In animals immunized with bovine serum albumin, 5-hydroxytryptophan prolonged the latent period of the IgM and IgG primary responses, decreased response intensity, delayed the response peak and suppressed IgG immunological memory. In 5-hydroxytryptophan-treated mice, the number of rosette-forming cells (RFC) in the lymph node and spleen decreased during the primary and secondary responses. This effect was due to the decreased number of IgG RFC and to the later involvement of IgM RFC in the immune response. The absence of the secondary response was related to unprimed IgG memory cells. 5-hydroxytryptophan does not inhibit the primary and secondary responses after the connections between the hypothalamus and pituitary have been disrupted. The participation of the n.raphe-hypothalaminc-pituitary system in immuno-regulation and the putative mechanism underlying serotonin effect on the immune response are discussed.