Henry Taylor and Ryan Olowofela both scored on their Franklin's Gardens bow but Saints were beaten 34-26 in a lively pre-season game against Leinster.
The home side had headed in at half-time 22-7 down, but after making a raft of changes they produced a huge response in the second half.
Scrum-half Taylor, a summer signing from Saracens, started it off with a score seconds into his first Saints appearance, and former England sevens wing Olowofela and Academy graduate Tommy Freeman were also able to cross the whitewash in the second period.
But Leinster, who had gone behind for the first time at 26-22 down, issued a late riposte, scoring twice to ensure they would finish their pre-season campaign with a 100 per cent record.
For Saints it was a useful workout ahead of next Saturday's Premiership Rugby Cup opener at Sale Sharks.
And there were some hugely promising signs, especially as a couple of their tries came from trademark free-flowing moves.
What will also have impressed boss Chris Boyd was the way his side bounced back after being under early pressure.
Leinster had looked strong from the start, winning the ball in their own half after James Grayson got the game underway.
After camping in the Saints half for a few minutes, having initially come close to scoring, Leinster eventually got over as No.8 Caelan Doris picked up from a scrum and dotted down.
Wing Fergus McFadden converted, but Saints responded really well, winning a penalty in midfield and then going to the corner to crank up the heat.
But Leinster stood firm to stop Saints' slick interplay having an effect.
And it was the away side who were able to score again as some lovely movement from left to right opened the door for outside centre Jimmy O'Brien to race in.
It wasn't long before lively Leinster scored again as they knocked loudly on the door before blowing it down thanks to Hugh O'Sullivan's assist and Peter Dooley's try under the posts.
The Saints fans were desperate for something to shout about, and they got it thanks to a scything move that started with Connor Tupai's break and ended with Ollie Sleightholme kicking ahead for the speedy Tom Collins to score.
Grayson converted with aplomb before a big scrum battle took place, with Saints winning a penalty from the first close to the Leinster line, and the away side winning the second, allowing them to clear.
And after Grayson got caught isolated on the edge of his own 22, Leinster won a penalty, which fly-half Ross Byrne, on for the injured Ciaran Frawley, kicked.
Saints were keeping Leinster on their toes at the other end of the field, with Sleightholme again having an impact before Fraser Dingwall was met with a blue wall in the 22.
Both sides made wholesale changes for the second half, with Teimana Harrison coming on to skipper Saints.
And Harrison made an instant impact, picking up a superb offload from Taqele Naiyaravoro and charging forward before giving the ball to Taylor for his first Gardens try.
Tommy Mathews missed the conversion, but he was soon making amends with a more successful effort after Saints scored another delightful try.
Fraser Strachan, Reuben Bird-Tulloch, Taylor, with a lovely offload, and young full-back Freeman were all involved, with Freeman putting the finishing touches to it.
There were now just three points in it and some of Saints' new players were making a real impression, especially JJ Tonks, who produced some huge hits in the six shirt.
Leinster were struggling to get out of their own half and after Olowofela was stopped just short by a brilliant last-ditch tackle, he eventually got over, flying into the line and over the whitewash.
Mathews converted and Saints led for the first time at 26-22 up.
But Leinster weren't behind for long as a clever crossfield kick put them within range and Rory O'Loughlin finished things off.
Byrne missed the conversion to leave Saints just a point behind, but Leinster continued to push on and Connor O'Brien put the icing on the cake with a late score.
Saints: Tom Collins (Freeman 40); Ollie Sleightholme (Gillespie 40), Fraser Dingwall (Bird-Tulloch 40), Fraser Strachan (Olowofela 62 (Dingwall 68)), Ryan Olowofela (Naiyaravoro 40); James Grayson (Mathews 40), James Mitchell (Tupai 21 (Taylor 40 (Tupai 78))); Alex Waller (c) (Davis 21 (Franks 40 (Trinder 62))), James Fish (van Vuuren 40), Paul Hill (Garside 21 (Painter 40)); Devante Onojaife (Bean 40), David Ribbans (Coles 40); Tom Wood (Tonks 40), Ollie Newman (Harrison (c) 40), Mitch Eadie (Uru 40).
Leinster: Hugo Keenan (J O'Brien 62); Fergus McFadden (A Byrne 50), Jimmy O’Brien (O'Loughlin 40), Joe Tomane (C O'Brien 40), Cian Kelleher (Kearney 40); Ciarán Frawley (Ross Byrne 28), Hugh O’Sullivan (Gibson-Park 40); Peter Dooley (B Byrne 66), Rónan Kelleher (Milne 66), Michael Bent (Salanoa 66); Ross Molony (c), Ryan Baird; Max Deegan (Doris 62), Scott Penny (Connors 50), Caelan Doris (Fardy 40).
Referee: Ian Tempest