Harlequins have made nine changes to their starting XV for their final Champions Cup group game on Friday as they seek to finish top of Pool B.
With a place in the last 16 secured thanks to a dramatic win over Cardiff last week, England stars Marcus Smith and Joe Marchant are on the bench.
Tommy Allan plays at fly-half after two months out with a shoulder injury, with Scott Steele starting at scrum-half.
Huw Jones, Andre Esterhuizen and Nick David also come into the backs.
Hooker Jack Musk, prop Simon Kerrod, second row Hugh Tizard and openside flanker Luke Wallace start in the pack.
Senior coach Tabai Matson said: "It's a great opportunity for guys who have been working furiously for months now to play at the Stoop in front of a sold-out home crowd and cement our European aspirations."
Finishing top of the pool would mean home advantage at the quarter-final stage.
A bonus-point win would all-but secure that but Quins will have to wait for the Leicester Tigers-Bordeaux game with the two Premiership clubs only separated by points difference.
Castres can still qualify for the last 16 but have lost their last three Champions Cup matches, including a narrow 20-18 defeat by Harlequins in December.
Line-ups
Harlequins: David; Lynagh, Jones, Esterhuizen, Murley; Allan, Steele; Marler, Musk, Kerrod, Lamb, Tizard, Taulani, Wallace, Dombrandt (capt)
Replacements: Riley, Garcia Botta, Wilson, Hammond, White, Gjaltema, Smith, Marchant
Castres: Larregain; Zeghdar, Aguillon, Cocagi, Nakosi; Botica, Kockott; Tichit, Barlot, Guillamon, Vanverberghe, Whetton, Champion de Crespigny, Kafatolu, Kornath
Replacements: Ngauamo, Nostadt, Tierney, Hannoyer, Raisuqe, Fernandez, Le Brun, Botitu
Referee: Ref: Mike Adamson (Scotland)
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