The protozoan Bonamia ostreae (Ascetospora), a paasite of the flat oyster Ostrea edulis, was purified by differential and isopycnic centrifugations. Mice of the strain Balb/c were immunized with purified parasites and hybridomas were prepared by fusion of immunized mouse splenocytes with the mouse myeloma cell line P3-X63-Ag8-653. From the fusion, 12 clones were isolated and saved. The resulting antibodies were characterized with a solid phase radioimmunoassay (RIA). Antibodies from 7 clones reacted only with B. ostreae and not with normal host tissue. Although some of the antibiodies identified. One of these antibodies (20B2-1B12) has an apparent association constant of ca 3 X 10(8) M-1 and bound to a number of different sites on the parasite. These monoclonal antibodies should prove to be of great value as diagnostic and research tools.