The androgenic gland hormone (AGH) is known to control sex differentiation in crustaceans. AGH was purified from isolated androgenic glands (AGs) of the male isopod Armadillidium vulgare by three steps of reverse-phase high performance liquid chromatography (RP-HPLC) and its chemical properties were examined. The purified AGH-active fraction showed masculinizing activity when 38 pg of this preparation was injected into a young female of the same species. Only 160 ng of the material was obtainable from 2000 animals at about an 11% rate of recovery. The elution of AGH activity by molecular sieve HPLC indicated that molecular weight of AGH was 11,000 ∼ 13,000. AGH was inactivated by treatment with trypsin or by reductive carboxymethylation. The AGH activity was not affected by heat treatment at 100°C for 3 min. These results indicated that AGH was a heat-stable protein with disulfide bond(s).