Studied were the activity and properties of alkaline phosphatase (AP) in the seminal plasma, spermatozoa washed with physiologic saline, testes, and accessory sexual glands of a bull. The AP activity was highest in the seminal plasma (2493.2 IU/l) and lowest in the Kupffer gland (257.3 IU/kg crude tissue, on an average). In washed spermatozoa it proved by 10 per cent lower than the activity in seminal plasma. At 56 degrees C as much as 60-80 per cent of the AP in the investigated materials was inactivated for 30 min. AP was shown to be inactivated strongly (up to 84-97 per cent) by urea (3.8 M). L-arginine (10(-2) M) and EDTA (10(-3) M) inactivated to an equal extent AP in all studied organs. L-phenylalanine inactivated AP weakly (13-15 per cent) in the testis and the epididymis, and more strongly (32-57 per cent) in the accessory glands and the plasma. Agar electrophoresis revealed three to four isoenzymes of AP in the seminal plasma, three isoenzymes in the testis and the epididymis, and two isoenzymes in the accessory sexual glands and in spermatozoa washed with physioogic saline.