Northampton reached the Champions Cup quarter-finals after a battling 49-41 win over Castres.
French side Castres raced into a 12-0 lead after only 10 minutes through tries from Christian Ambadiang and Theo Chabouni, before Saints were awarded a penalty try with the lively Ambadiang shown a yellow card for a high tackle.
Castres were reduced to 13 players when lock Guillaume Ducat caught Henry Pollock in the ruck to receive another yellow, but they increased their lead with a penalty before Danilo Fischetti went over as Saints went in at half-time trailing 15-14.
The hosts led for the first time five minutes into the second half when Archie McParland's pass sent Josh Kemeny over, and the lead was increased to 13 when Tommy Freeman got on the scoresheet.
The deficit was back to one after tries from Tyler Ardron and Vuate Karawalevu, but scores by George Furbank and McParland put Saints back in charge.
Vilimoni Botitu reduced the arrears to eight, but Castres lost Veresa Ramototabua to the sin bin and Northampton took advantage when Fraser Dingwall crashed over.
Ambadiang had the last word with a final-minute burst from halfway, but Saints were home and hosed by then as they booked a tie with either Bath or Saracens in the last eight.
