The phenol oxidase enzymatic system (EC 1.10.3.1, EC 1.10.3.2) is widespread in different species of the animal and vegetal kingdom. Despite its importance in the eggshell formation of the trematodes phenol oxidase (PO) has been little studied in these organisms, mainly in S. mansoni. This report presents the initial results concerning the immunization of rabbits with PO of S. mansoni and mushroom tyrosinase. The immunological analysis done by means of double immu-nodifusion (Ouchterlony) and immunoelectrophoresis techniques revealed some immunological identity between the PO of males and females. It was not seen cross reaction between the antisera against PO and tyrosinase, what suggests that the antigenic determinants of both enzymes are different in spite of their catalytic sites being similar, since they act over the same substrate. The results reported here represent a first step in way to obtain the PO isoenzymes in their pure form and should open new insights for further studies on the molecular mechanisms involved in the sclerotization process of the S. mansoni eggshell.