The murine monoclonal antibody (MoAb) CB21, raised after immunization with sonicated extracts of human platelets, has been shown to react with a line-restricted surface molecule and also a cytoplasmic structure displaying no restriction in terms of lineage and species. The surface structure recognized by the CB21 MoAb is exclusively expressed on the surface membrane of human platelets, being undetectable on other cells or lines so far tested. After permeabilization, the majority of the cells and lines tested with the CB21 MoAb displayed strong cytoplasmic reactivity with a constant typical filamentous distribution. Biochemical and morphological analyses showed that the cytoplasmic counterpart recognized by the CB21 MoAb is the intermediate filament type III.