An incidental observation initiated this study on the effects of prostatic massage on the serum acid phosphatase level in 20 patients. In 8 patients the level rose from normal values to above 0.7 units and in 4 of these patients it reached definite "carcinoma" levels. It is concluded that acid serum phosphatase detns. should not be performed soon after a prostatic massage. Reliable and accurate values will be obtained 24 hrs. after massage, if the patient''s kidney function is not impaired.