The fine structure of the process of interaction between uncoated and antibody-coated Tritrichomonas foetus with rat peritoneal eosinophils was studied. Eosinophils were purified by discontinuous Metrizamide gradient. Agglutination and immunofluorescence microscopy showed that T.foetus grown in a medium supplemented with different kinds of serum, antibodies present in it opsonize the parasites. Parasites incubated in the presence of anti-T.foetus rabbit serum attach to and are ingested by eosinophils. No such attachment was observed with trypsin-treated parasites. Attachment of antibody-coated parasites to the eosinophils induced their degranulation with the release of granule contents onto parasite surface causing its destruction. Hyperimmune serum was shown to be required for both attachment and ingestion of the parasites. The cytochemical localization of basic proteins and peroxidase showed that leucocyte granules fused with parasite-containing phagocytic vacuoles. Images were obtained suggesting that the binding of the parasites to the eosinophil surface triggers an exocytic process with release of the granule content onto the parasite surface.