Mullerian inhibiting substance (MIS), a fetal testicular product which causes regression of the Mullerian duct in he male mammalian embryo, is being evaluated as an inhibitor of genital tract tumors. The present study demonstrates in a clonogenic soft agar assay that biologically active MIS inhibits the growth of a human ovarian carcinoma cell line (HOC−21) in a dose-dependent manner when compared to phosphatebuffered saline medium controls, heat-inactivated MIS, or biochemical fractions which lack biological activity (P < 0.05). (J Clin Endocrinol Metab54: 1051, 1982)