An automated assay of anti-P1 allohemagglutinins has been carried out on 133 P2 subjects: 13 with acute hepatic distomiasis (Fascioliasis), 20 with hydatidosis and 100 healthy blood donors. Anti-P1 were detected in 100% of the distomatid sera, in 50% of the hydatid and in 29% of the healthy individuals. When compared to a reference serum test (anti-P1 standard), their concentration was found to be weak in healthy subjects, moderate in hydatidosis and exceptionally increased in acute distomiasis (up to 5-6 times the standard anti-P1 level). The 2-mercaptoethanol treatment showed that even the increased distomatid anti-P1 sera were of an IgM nature. The respective origin, synthesis and strength of these allohemagglutinins are discussed and the advantages of using distomatid IgM anti-P1 as human sera test are emphasized.