Binding to Trypanosoma brucei of polyvalent IgMs and IgGs, monoclonal IgGs and Fab1 fragments of monoclonal IgGs specific for exposed epitopes of T. brucei variant surface glycoproteins (VSGs) was monitored by both immunofluorescence and immunocytochemistry. All antibodies and antibody fragments, were uniformly distributed over the parasite surface after incubation with the organism at 0 degrees C. Upon warming to 37 degrees C bound antibodies and fragments were detected in the flagellar pocket and intracellular organelles. Removal of single layers of bound antibody, or Fab1 fragments, from the cell surface at 37 degrees C, as determined by immunofluorescence, was complete within 20 min and occurred in the presence or absence of protein synthesis. Parasites that had shown an altered distribution of surface-bound antibody after warming remained fully covered with VSGs of the original antigen type as shown by immunocytochemistry.