Alkaline phosphatase activity in several cultured primary human intracranial tumor cells varied over a relatively wide range, and there was no correlation between specific activity and the type of tumor from which the cultures were derived. The enzyme was thermolabile, and its activity was strongly inhibited by l-bromotetramisole, levamisole, and L-homoarginine but not by L-phenylalanine and L-phenylalanyglycylglycine. These are the characteristics of the liver-bone-kidney form of alkaline phosphatase. Prednisolone induced increased levels of enzyme activity in most cultures, and sodium butyrate acted as an inducer in cultures of pituitary adenoma and hemangioblastoma cells. The increase was most pronounced when response cells were exposed to both stimuli simultaneously. The induced alkaline phosphatase had the same properties as the enzyme of cells grown in the absence of inducers. Increased alkaline phosphatase activity was not induced by osmolality changes of the culture medium; this feature appears to be characteristic of cells producing the liver-bone-kidney enzyme form.