Epithelial cells cultured from normal human endometrium can be maintained for prolonged intervals in vitro and can be induced to differentiate in response to variations of the hormone content of the culture medium that simulate the menstruation cycle. In response to repetitive treatments with N-methyl-N'-nitro-N-nitrosoguanidine these cells progress through a sequence of morphologic and biologic alterations which culminate in characteristics, like the ability to grow as colonies in soft agar, that have frequently been associated with malignant transformation. The ability to analyze the transitions between sequential stages of progression in this system provides the means to study the biologic significance of the various stages and the transitions between them. This cell culture system will be used to distinguish whether a phenomenon analogous to promotion in two-step carcinogenesis in vivo, can be demonstrated to occur in vitro. If a strong promoter (for skin carcinogenesis) is active in this system, then the transitions between stages of the progression toward malignant transformation can be analyzed to distinguish transitions caused by this promoter from these that require an initiator or complete carcinogen. Similarly, the transitions that specifically require a strong promoter can be distinguished from those that can be caused by an incomplete promoter (i.e., an agent that suffices to complete promotion following single or limited treatments with a strong promoter). Studies will also evaluate estrogens and inflammatory cells (or their extracellular products) because they are plausible natural promoters for endometrial cancer in humans, and progestins because they are presumed inhibitors of endometrial cancer. By analysis of the effects of these agents on transitions during progression, their specific effects may be determined, and estrogens and inflammation may be classified as complete or incomplete promoters. On a practical level this information could provide a rationale for the use of progestins as chemopreventive agents for endometrial cancer, or conversely, this information could delimit the appropriate use of estrogens to avoid promoting effects. On a basic level this information may offer insight concerning the critical effects of promoters and this in turn could guide future studies to identify the mechanism of promotion.