Unlike other eukaryotic cells, Trypanosoma lewisi forms caps at 0 degrees C when incubated with rabbit immunoglobulin G(IgG) directed against surface IgG from the rat host. The host IgG, which is specific for parasite antigens, probably does not cause capping of these antigens in vivo, since trypanosomes treated with Fab fragments directed against rat IgG are uniformly labeled and do not cap at 0 degrees C or 37 degrees C.