A micromethod for the determination of alkaline phosphatase activity in a few cells from two cell lines, undifferentiated embryonal carcinoma F 9 cells and differentiated embryonal mouse fibroblast STO cells, is presented. In F 9 cells a 4- to 5-fold higher activity of alkaline phosphatase than in STO cells extrapolated from 10 cells was evaluated and compared with the results of the specific activity from cell extracts of these two cell lines. The significance of the results and the application of the method is discussed.