5-Hydroxytryptamine (5-HT, serotonin)-containing cells were localized in the central nervous system of Octopus vulgaris by use of the unlabelled peroxidase-antiperoxidase complex (PAP) immunohistochemical method employing highly specific antibodies to 5-HT present in paraformaldehyde-fixed tissue. Antibodies were raised in rabbits against an immunogen prepared by coupling 5-HT to bovine thyroglobulin (BTG) or to bovine serum albumin (BSA) with formaldehyde as the coupling reagent. The specificity of the immune reaction was studied by both absorption test and radioimmunoassay. The distribution of 5-HT immunoreactivity observed in octopus brain was essentially similar to that reported by other workers who used formaldehyde- or glyoxylic acid-induced fluorescence method. In addition, this immunohistochemical technique revealed 5-HT-containing perikarya in both the chromatophore and the palliovisceral lobes which were not detected by the previous fluorescence histochemical method. Thus, this immunocytochemical procedure appears to be a specific and very sensitive technique for the localization of 5-HT within the central nervous system of cephalopod Mollusca.