The role of cytoskeletal microfilaments in the cap formation of surface immunoglobulin (sIg) of rabbit spleen and peripheral blood B lymphocytes was investigated using inhibitors such as colchicine, cytochalasin B, cytochalasin D or combinations of these drugs. By immunofluorescence combined with phase contrast microscopy, analysis and sorting with a fluorescence activated cell sorter, combined staining of membrane-bound and cytoplasmic immunoglobulin, and electron microscopy, it could be established that spleen B lymphocytes consist of two subpopulations with different capping behaviour: 1. A majority of small (5-7 micrometer diameter) lymphocytes with the morphology of resting cells. This cell type was unable to form caps ("non-cappable"), and formed patches which rapidly underwent endocytosis. In these cells a connection of sIg with microfilaments is postulated, as they were triggered to cap formation in the presence of cytoskeletal inhibitors. 2. A minority of large (6-13 micrometer diameter) lymphocytes with morphological features of slightly activated cells. These cells display cap formation independent of microfilament activity. Cytoplasmic staining revealed that, although they should be considered to be rather differentiated by their age-dependent occurrence, they do not belong to the plasmablast-plasma cell series. These results demonstrate that cytoskeletal microfilaments in rabbit B cells do not play a role in generating the driving force of sIg cap formation.