Capping of surface Ig by anti-Ig antibodies involves a membrane perturbation requiring an energy-dependent step. Lymphocytes treated with anti-Ig are stimulated to move. Previously, we had shown that movement was not essential for capping, although it influenced the localization of the cap. We have investigated the role of cell movement and of microtubular proteins in this phenomenon. Treatment of B lymphocytes with colchicine does not affect capping of Ig nor does it affect the increase in translational movement produced by anti-Ig antibodies. Treatment of lymphocytes with cytochalasin B stops translational movement and may affect capping to some degree under appropriate circumstances. Lymphocytes treated with both drugs are impaired in capping. We surmise that there may be two cytoplasmic events regulating directly or indirectly capping: one associated with the process of translational movement, the other associated with the activity of microtubules. Lymphocytes treated with concanavalin A do not cap Ig. Colchicine reverses this inhibition. Certain experimental procedures antagonize the colchicine effect, the most striking of which is the use of cytochalasin B. Colchicine appears to increase movement of the Con A-treated lymphocyte, and this increased movement appears responsible for the accumulation of complexes to the posterior part of the cell. Con A inhibits patching of Ig by anti-Ig, and this is not reversed by colchicine.