Cellular ras genes encode a family of membrane-associated proteins (p21ras) that bind guanine nucleotide and possess a low intrinsic GTPase activity. The p21ras proteins are ubiquitously expressed in mammalian cells and are thought to be involved in a growth-promoting signal transduction pathway; their mode of action, however, remains unknown. The ligand-induced movement of cell-surface receptors seems to be a primary event in the transduction of several extracellular signals that control cell growth and differentiation. In B lymphocytes, surface immunoglobulin receptors crosslinked by antibody or other multivalent ligands form aggregates called patches, which then collect into a single assembly, a cap, at one pole of the cell. This process constitutes the initial signal for the activation of a B cell. Here we show by immunofluorescence microscopy that p21ras co-caps with surface immunoglobulin molecules in mouse splenic B lymphocytes. In contrast, no apparent change in the distribution of p21ras occurs during the capping of concanavalin A receptors. The redistribution of p21ras is apparent at the early stages (patching) of immunoglobulin capping and is inhibited by metabolic inhibitors and the cytoskeleton-disrupting agents colchicine and cytochalasin D. The distribution of another membrane-associated guanine nucleotide-binding regulatory protein, the Gi alpha subunit, is not affected by surface immunoglobulin capping. These findings demonstrate that p21ras can migrate in a directed manner along the plasma membrane and suggest that p21ras may be a component of the signalling pathway initiated by the capping of surface immunoglobulin in B lymphocytes.