Antiserum was raised in a rabbit against the purified androgenic gland hormone (AGH) ofArmadillidium vulgare.The specificity of the antiserum was confirmed by immunoblotting and by absorption of the biological activity of AGH. The immunological activity measured by ELISA correlated well with the biological activity of AGH. It is therefore possible to measure AGH by immunoassay. The immunoassay, which is time-saving and avoids the handling problems of the bioassay was used to measure seasonal variation of AGH content in male reproductive organs. Lack of immunological crossreactivity between preparations from different isopod species supported former results on biological species-specificity.