The electrophoretic pattern (in polyacrylamide gel) for acid phosphatases in the prostate gland was compared with that for other tissues. Isoenzyme 2 predominates in the prostate. The isoenzyme was isolated from the prostate and its biochemical properties were compared with those of acid phosphatases isolated from spleen. Isoenzyme 2 has a molecular weight of about 100,000. Its optimum pH is between 5 and 7, unlike other lysosomal enzymes. Its substrate specificity is not very much different from those of the most active isoenzymes of acid phosphatase in other tissues. Our results contraindicate the use of a specific substrate in the analysis of prostatic acid phosphatases. Determination of the isoenzyme pattern is a new approach in the specific analysis of prostatic acid phosphatases.