Prostaglandin inhibitor--indomethacin was found to significantly suppress the growth of 5 epithelial transplantable tumors in mice. Simultaneously, it stimulated the development of leukemias L-1210 and La and intraperitoneal melanoma B-16. It was also shown to stimulate the spontaneous development of leukemia in mice AKR. Its growth-inhibiting effect was in direct correlation with the rate of prostaglandin E2 synthesis in tumor. The data obtained point to a specific metabolism of polyunsaturated fatty acids in experimental tumors of varying histogenesis as well as the importance of prostaglandin E2 level for prognosis of the inhibitive effect of indomethacin.