Cell cultures of a differentiated kidney epithelial cell line, MDCK, spontaneously form fluid-filled domes or hemicysts composed of numbers of cells as a manifestation of specialized epithelial transport phenomena. Addition to MDCK cells of a broad spectrum of compounds that are known as potent inducers of mammalian cell differentiation in cell culture caused a striking increase in the frequency of dome formation. Polar compounds such as N,N-dimethylformamide, dimethyl sulfoxide, or hexamethylene bisacetamide stimulated increased dome formation 15--30 hr after addition. Induction of domes by these compounds was prevented either by inhibitors of protein synthesis or by ouabain, cytochalasin B, or vinblastine. Inhibition of DNA synthesis did not block chemical induction of domes. Other inducers were compounds of physiological occurrence such as n-butyrate or adenosine. Furthermore, a variety of conditions expected to elevate intracellular levels of cyclic AMP also stimulated dome formation. These findings suggest the hypothesis that domes are formed in cell culture by a form of cell differentiation that is under positive control by cyclic AMP.