A humoral immune response of the Japanese eel (Anguilla japonica) against major antigens from the microsporean Pleistophora anguillarum was detected. Sonicated spores recovered from the skeletal muscle of infected "beko" eels were run on sodium-dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions. The separated proteins were then electroblotted onto nitrocellulose and incubated in dilutions (1: 100, 1: 200 and 1: 300) of sera from infected or non-infected eels. For secondary and tertiary incubations, a rabbit anti eel Ig serum and peroxidase conjugated swine anti rabbit Ig antibodies were used, respectively. The reaction pattern varied among eels, but it was demonstrated that the sera of infected eels responded strongly to at least one of the proteins with approximate molecular weights of >94, 89, 62, 52, 45, and 32 kDa. Some sera of non-infected eels from the same culture ponds showed a lower reactivity to the >94, 62, 52 and 32 kDa proteins (serum dilution 1: 100) but no sera responded to any proteins when they were diluted 1: 200 and 1: 300.