The role of serotonin receptors in the inhibitory effect of serotoninergic system on immunogenesis was studied using cyproheptadine, a specific blocker of 5-HT2 receptors. It was shown that cyproheptadine administration to CBA mice stimulated the immune response, which was dopamine-dependent and was realized via thymus. With the pituitary stalk destruction, the stimulatory effect of cyproheptadine was not observed, which suggests the participation of 5-HT2 brain receptors in immunogenesis.