Sera from Balb/c mice, hyperimmunized with ruptured epimastigotes of either Trypanosoma cruzi or Trypanosoma rangeli, lacked species-specificity when assayed for antibodies by indirect immunofluorescent microscopy and by the enzyme-linked immunosorbent assay. However, when plasma cells from those mice were fused with syngeneic mouse plasmacytoma cells, many of the resultant hybridomas synthesized antibodies which were species-specific. Four clones are synthesizing antibodies specific for antigens of T. rangeli. These antigens are associated with the cytoplasm, plasma membrane and flagellum. One clone is synthesizing a specific anti-T. rangeli antibody which appears to be reactive with the entire surface of the epimastigote. Another clone is reactive with a subpopulation of epimastigotes, thus suggesting some antigenic variability among cultured forms. Two clones are synthesizing antibody specific for antigens of T. cruzi, one of which is confined to the cytoplasm.