The testing of the type of membrane associated alkaline phosphatase (EC 3.1.3.1) by immunotitration has revealed that there is a shift from a liver-like type of alkaline phosphatase in normal cell cultures of the human diploid fibroblast cell strains WI 26 and WI 38 to a placenta-like variant in cultures of the same cell strains after the transformation by the DNA-tumor virus SV 40, the WI 26 SV 40 and the WI 38 SV 40 cell lines. The immunologically detectable switch-over has been confirmed by measuring the apparent Michaelis constant and the heat stability of the AP from normal and transformed cells. Liver-like AP is heat labile and has an apparent Michaelis constant for p-nitrophenylphosphate (about 4.0 X 10(-4) M). The placenta-like AP shows heat stability and a lower apparent KM (about 2.2 X 10(-4) M). The appearance of the so-called Regan enzyme of AP in some human tumors in vivo is discussed in this connection.