Human blood lymphocytes contain two populations of immunoglobulin-bearing cells. One has passively acquired, labile IgG on its surface, is not phagocytic, and does not contain detectable peroxidase. It also does not show capping or stimulation of motility after binding anti-IgG antibody. The other population of true B cells has IgM and IgD on its surface, most cells having both. This population behaves as conventional B lymphocytes, showing capping and anti-immunoglobulin-stimulated motility. The capping is independent of cell motility and cannot be inhibited by colchicine. The effects of anti-IgM and anti-IgD on these cells are similar in that they both stimulate capping and motility to about the same extent and both cap to the same region of the cell surface.