Trypanosoma brucei subsp. brucei and T. congolense incubated in homologous antibody at 0 degrees C for 30 min were lysed by subsequent addition of guinea pig complement. Trypanosomes incubated in antibody at 37 degrees C before complement treatment remained intact. Parasites bearing adsorbed antibody also remained intact when incubated at 37 degrees C before complement treatment. The proportion of cells which survived complement treatment decreased with increasing antibody concentration. Parasites which survived complement treatment continued to express antigens which could bind complement-activating antibody, but did not bear complement-activating immune complexes. Virtually all cells in T. brucei populations exposed to antibody at 37 degrees C, but only 10(-3) to 10(-5) of the cells in populations exposed to antibody at 0 degrees C before complement treatment, remained infective for X-irradiated mice. Only 10(-6) to 10(-7) of T. brucei populations exposed to antibody at 0 or 37 degrees C before complement treatment infected mice immunized with homologous antigens. Serotype analysis of substrains and of T. brucei populations isolated from mice infected with antibody and complement-treated parasites suggested that variant-specific antigens participated in trypanolysis and that T. brucei which survived complement treatment could undergo antigenic variation. Mechanisms by which trypanosomes may inactivate or eliminate surface immune complexes and the possible significance of this phenomenon in trypanosomiasis are discussed.