Abstract H6 embryonal carcinoma cells form aggregates of cells in culture medium which contains 2 m M calcium. These aggregates are described as uncompacted, indicating that the individual cells of the aggregate are spherical and are in limited contact with each other. In contrast, compaction of the aggregate, induced by increasing the calcium concentration, results in a tight mass of cells flattened against one another and connected by intercellular junctions. At least 85–97% of the aggregates undergo compaction in 7 m M calcium and are subsequently decompacted if removed to 2 m M calcium. Since calcium ionophore A23187 does not induce compaction, extracellular rather than intracellular calcium seems to be the limiting factor. We have demonstrated that this calcium-induced morphogenetic change is sensitive to inhibition by agents which also prevent the calcium-dependent compaction of the 8-cell mouse embryo. The cytoskeletal-binding drugs tetracaine HCl, colcemid, vinblastine, colchicine, and cytochalasin B each inhibit compaction of H6 aggregates. Interference at surface molecule sites by exposure to the lectins wheat germ agglutinin or concanavalin A or by interruption of glycosylation with exposure to tunicamycin, or by reaction with anti-H6 Fab or anti-F9, also prevent compaction. Since the mouse embryo and embryonal carcinoma cells share certain processes which are involved in initiating and maintaining compaction, these processes and their subsequent roles in differentiation may be examined using embryonal carcinoma cell aggregates.