The effects of progesterone on the growth and differentiation of human endometrial adenocarcinoma cells (cell lines SNG-P and SNG-M derived from primary and metastatic tumors, respectively) were assessed in vitro and in vivo. Progesterone suppressed their growth and induced cell differentiation in vitro. The suppressive effect of progesterone was stronger in the primary tumor cells than in the metastatic ones. Progesterone produced morphologic changes such as multinucleation, multinucleolation, vacuolation, extensive Golgi apparatus, and papillary arrangement of cells. The cells were transplanted sc into nude BALB/c mice where they produced undifferentiated adenocarcinomas in untreated mice and well-differentiated adenocarcinomas in progesterone-treated ones. Progesterone reduced tumor growth and decreased transplantability in nude mice. This hormone produced no change in the distribution of the chromosome numbers or in the karyology.