The sea robin (Prionotus evolans), a representative teleost fish, was found to undergo the hapten-carrier effect, suggestive of T-B cell cooperativity, in vivo with dinitrophenyl as the hapten and bovine γ-globulin or bovine serum albumin as carriers. In contrast to mammalian systems, the secondary anti-hapten response in the sea robin was found to involve exclusively high molecular weight antibody.