We have re-evaluated the isolation and characteristics of human urinary alkaline phosphatases (ALPs). From the results of physicochemical properties and immunological identification, the urinary ALPs from healthy subjects and patients with hepatoma were found to be similar in nature to liver and/or bone-like ALP. In patients with chronic or acute nephritis, the ALPs contained a major band of kidney-like ALP with a minor band of bone and intestinal ALPs. However, the ALPs in pregnant women had not only liver and bone ALPs but also placental-like ALP. It is interesting that only bone-like ALP was detected in psychiatric patients administered chlorpromazine. In the conditions we investigated, the molecular sizes of the urinary ALPs were similar as those of original ALPs, except for the enzyme from renal failure. Moreover, the total activity of urinary ALP was closely related to the level of serum ALP, being in a ratio of 1/40. In general, urinary ALP may be derived from serum ALP by minor modification, suggesting that the identification of excreted ALP in urine is a good marker for disturbed organs in respective diseases.