Ninety-six percent of the P. aragonimus uterobilateralis patients and 86.5% of the P. africanus patients showed a positive reaction with the homologous antigen in the passive hemagglutination test (PA). Common antigens of the 2 species were demonstrated by cross reactions. About 75% of the Paragonimus sera reacted with the heterologous antigen with titers .gtoreq. 1:160. The PA test can be useful in the evaluation of the efficacy of treatment and is well suited for seroepidemiologic purposes. The complement fixation (CF) test was not suitable for serodiagnostic or seroepidemiologic studies nor for the assessment of the cure of African paragonimiasis since antibodies persisted for at least 1 yr after treatment. Comparative studies on levels of IgA [immunoglobulin A], IgE, IgG and IgM seem to be of no diagnostic value in African paragonimiasis because no significant correlation between Ig concentration and paragonimiasis could be recognized. Very high IgE levels (up to 100 times higher than in Europeans) were found in nearly all sera. Shipment of filter paper blood specimens collected from P. africanus patients resulted in a significant decrease of antibody reactivity in all sera, the loss being about 30% in the parasitologically proven patients.