The enzymological and immunological properties of alkaline phosphatase (ALP) isoenzymes in intestinal metaplasia and gastric carcinoma tissues were compared with those of purified human intestinal, placental, and hepatic ALP's to speculate the gene expression of gastric cancer cells. The results were as follows: Intestinal metaplasia was found in 21 of 23 cases (91%) of gastric cancer, whereas normal gastric mucosa had no ALP activity. ALP extracted from the mucosa of intestinal metaplasia was identical with intestinal ALP as to enzymological and immunological properties. ALP obtained from gastric carcinoma tissues of 23 patients was separated into 3 bands (ALPa, ALPb, and ALPc) by polyacrylamide-gel disc electrophoresis. Ten cases showed ALPa only, 4 cases showed ALPa and ALPb, 5 cases showed ALPa and ALPc, and the other 4 cases showed ALPa, ALPb, ALPc, respectively. It was concluded that ALPa was similar in its enzymological and immunological properties to hepatic-type ALP, ALPb was similar to placental-type ALP, and ALPc was similar to intestinal-type ALP, ALPb probably originates from the cancer cell itself. As for the other enzymes, ALPa and ALPc, further investigation should be made to determine whether they come from the cancer cell itself or not.