This is the history of discoveries of several enzyme tumor markers in the awardees laboratory. The first, beta-glucuronidase, was originally related to the physiological actions of estrogens and androgens. Perfection of histochemical techniques based on new substrates demonstrated the dual localization of beta-glucuronidase in endoplasmic reticulum and lysosomes. Tumor tissues, in general, are enriched with beta-glucuronidase. Next, acid phosphatase of the prostate gland possesses the distinctive property of undergoing inhibition by L-tartrate. This organ-specific inhibitor was incorporated into the Fishman-Lerner method for measuring serum acid phosphatase of prostatic origin. This significantly increased the specificity of the measurement of serum acid phosphatase for prostatic cancer. Finally, the discovery of the Regan Isoenzyme, placental alkaline phosphatase (PLAP) in a patient with disseminated lung cancer provided a tumor marker useful in the management of gonadal tumors, in particular. Closely related to PLAP is germ cell alkaline phosphatase which is eutopically expressed in seminoma. Finally, radioimmunolocalization and radioimmunotherapy of PLAP in these tumors have been achieved by others.