The immunomodulatory effects of phagocytic cells in common carp Cyprinus carpio L., by alpha melanocyte stimulating hormone (α-MSH) was analyzed in vitro. Carp head kidney leucocytes were cultured in RPMI 1640 medium containing 0.05, 0.1, 1 and 10nM α-MSH and the production of superoxide anion was measured via the reduction of nitroblue tetrazolium (NBT) in vitro. Macrophages incubated with α-MSH showed an increase in the production of superoxide anion in comparison to control macrophages cultured without hormone. Phagocytic cells treated with α-MSH also displayed increased phagocytosis. Furthermore, carp lymphocytes treated with α-MSH increased the mitogenic responses to phytohaemoagglutinin (PHA-P). These results show that α-MSH directly influences fish immune responses.