Serum phosphatase determinations have become important aids in diagnosis, particularly the diagnosis of prostatic cancer, but there has been no clear understanding regarding the interpretation of these tests under various conditions that are met clinically. Acid phosphatase is an enzyme which is found only in the epithelial cells of the prostate gland in sexually active men and in the cells of prostatic cancer. Normally there is a small amount of this substance in the blood serum. The enzyme owes its existence to the sustaining influence of the male sex hormone, and for this reason it has been termed a chemical secondary sex characteristic. After castration it disappears from the prostatic epithelium, and a coincidental shrinkage of the tissues occurs. This phenomenon was utilized advantageously in the treatment of prostatic carcinoma by Huggins, whose investigations inaugurated a new era in cancer research.1 It is well established that sustained elevation in