Abstract We have investigated the effects of castration and androgen replacement on the kinetic characteristics of rat prostatic acid phosphatases (AP). Chromatography on DEAE-cellulose DE-23 and on Sephacryl S-200 allowed the separation of lysosomal (S-1) and secretory (S-3) forms of AP. In addition, these techniques revealed a third enzymic form (S-2), which eluted in the void volume of the Sephacryl S-200 column and which occurred in significant amounts only 15 days after castration. The S-2 form was extremely resistant ( K 1 1500 μM) to L-tartrate inhibition in the 15-day post-castration rat which is in contrast to the behaviour of S-2 ( K i 145 μM) in control animals. The appearance of this unidentified AP form can explain the dramatic drop in the per cent inhibition of total rat AP observed between 7 and 15 days post-castration.