Cell locomotion involves several structural-functional activities: membrane extensibility, microfilament regulation and adhesive interactions. There is evidence for Ca2+ requirement in all of these. Our data may clarify the role of Ca2+ in locomotion and adhesion. Morphologic and spreading responses of isolated blastula--late gastrula Rana pipiens germ layer cells to varying molar concentrations of Ca2+; 0-Ca2+, Standard Ca2+ (Barth's X solution), 1.5 x and 2.0 x Std Ca2+ were viewed by S.E.M. after 1 h in culture. Ionic strength and pH were constant. All cells showed quantitative relationships between Ca2+ concentration and surface extensibility, projection formation and presumably adhesion, but with tissue- and stage-specific variations. Cells in Ca2+-free medium fail to adhere (50%), flatten or form surface projections. Cells in media with increasing Ca2+ generally formed more numerous and extensive surface projections, spread and adhered to a greater extent. In some cases there were no quantitative differences in response between 1.5x and 2.0x standard Ca2+. Cells in suspension for 1 h in standard solution remained spherical, forming no projections. We infer from these results that both Ca2+ and contact with a physical substratum, cell--cell of cell--glass are required for mobilization of the various systems involved in locomotion and adhesion. In addition, components of these systems are quantitatively activated by increased availability of Ca2+.