The early cycle of events which follows the binding of anti-Ig ligands to the surface Ig molecules of B lymphocytes was compared in several inbred strains of mice. Kinetic differences were found in the rate of surface Ig capping and absolute differences in extent were found for anti-Ig-induced B cell motility. These strain differences were not restricted to the B cell surface Ig molecules, however, since capping by Con A of its receptor sites on B and T cells and random T cell motility were also affected. A genetic analysis of A and CBA mice revealed that each of the traits studied is probably affected by two gene differences between the two strains analyzed. The possible roles of microtubules and cyclic nucleotides in these genetic differences were also explored. Colchicine, which depolymerizes microtubules, raised the low capping and motile responses of CBA spleen cells to the high responses characteristic of A strain cells. This implies an excess of microtubular function in CBA cells. Raising the level of cyclic AMP also raised the low CBA capping response to the high A level, but did not differentially effect the motile responses of cells of the two strains. The possible significance and locus of action of these effects is discussed.