Digestive glands of mussels Mytilus edulis from Brittany, France, infected with Marteilia sp. (Ascetospora) were used to purify the parasite. A modification of a previously used purification protocol increased purification efficiency, permitting sporangial primordia and sporangia of Marteilia sp. to be obtained. Mouse (Balb/c) monoclonal antibodies were generated against this parasite. From the fusion, 26 monoclonal antibodies against Marteilia sp. were obtained. Antibodies from 6 clones reacted only with Marteilia sp. cells and not with normal host tissues. Four of these antibodies (1/1-3, 3/1-1, 4/1-1 and 6/2-3) reacted with the sporangia wall and two with the spore cytoplasm (9/1-1 and 12/5-1). Antibodies cross-reacted with Marteilia refringens from Mytilus galloprovincialis obtained in the Ria de Vigo, Spain.