Northampton ran in five tries as they beat Castres for their first victory in this season's Heineken Cup campaign.
Last year's finalists Saints entered the game knowing they could no longer qualify for the knock-out stages after losing all three Pool One games.
Fly-half Stephen Myler's penalty was the only score in a quiet first half.
But tries from Ben Foden (2), Vasily Artemyev, Mikey Haywood and Greig Tonks after the break sealed a bonus-point victory for Jim Mallinder's men.
Northampton can still qualify for the European Challenge Cup if they hold onto third spot in the pool.
The game at Franklin's Gardens lacked any real bite as both coaches fielded under-strength sides in the knowledge that there was little resting on the game.
Saints were frustrated by Castres' limited game plan, with the French side slowing the game down and preventing their hosts from building any momentum.
Myler took his first penalty chance early on in the first half before Castres suffered a blow when Pierre Roussel was yellow-carded for pulling back Myler.
But despite having the better of the possession and territory plus the benefit of facing a side who were down to 14 men, Northampton failed to add to their score before the break.
Myler added three more penalties on resumption before Mallinder introduced scrum-half Lee Dickson on the hour in a bid to give his side more attacking variety.
The move paid dividends as Foden and Artemyev both outflanked the visitors' defence to score tries which Myler duly converted.
Castres were demoralised and conceded further tries to Haywood and Tonks before Foden ran in a final one from 50 metres to score.
Director of rugby Jim Mallinder told BBC Radio Northampton:
"It was a little bit frustrating in the first half - we had the majority of the play and a few opportunities but we didn't take them.
"But in the second half we stuck to our guns and it was a good performance in the end.
"I was delighted as well with the impact of our subs - I thought they came out and made a big difference."
Northampton: Foden; Artemyev, Clarke, Downey, Elliott; Myler, Roberts; Waller, Hartley, Doran-Jones, Lawes, Sorenson, Wood, Nutley, Manoa.
Replacements: Tonks for Clarke (68), Armstrong for Downey (61), Dickson for Roberts (61), Haywood for Hartley (62), Craig for Lawes (51), Ingle for Nutley (63).
Not Used: Tonga'uiha, Mercey.
Castres: Denos; Nicolas, Bonnefond, Garcia, Lakafia; Bernard, Lacrampe; Peikrishvili, Bonello, Kruger, Rolland, Murray, Roussel, Malonga, Bornman.
Replacements: Inigo for Nicolas (68), Kockott for Lacrampe (61), Ducalcon for Peikrishvili (52), Rallier for Bonello (52), Coetzee for Kruger (45), Capo for Rolland (68), Caballero for Bornman (43).
Not Used: Bai.
Sin Bin: Roussel (22).
Ref: George Clancey (RFU).