Immunoglobulin E (IgE), mast cells, and eosinophils in the skin of rhesus monkeys immunized with highly X-irradiated cercariae of Schistosoma japonicum were studied. IgE was stained by the unlabeled antibody enzyme method and was found on mast cells. Before challenge, mast cells were found only in the dermis. Immediately after challenge, mast cells were found both in the dermis and epidermis and many of them were degranulated. Soon after, margination and emigration of granulocytes, predominantly eosinophils, occurred along the blood capillaries in the dermis. Gradually, perivascular infiltration of eosinophils was seen in the dermis and migration of eosinophils from the dermis into the epidermis appeared, resulting in the formation of minute eosinophilic abscesses in the epidermis. In addition, the IgE was found as a thin coat on the integument of the schistosomula. Deteriorated schistosomula were seen amid the eosinophilic abscesses in the epidermis and in eosinophilic infiltrations in the dermis. The present findings suggest the possibility that the reactions of IgE, mast cells, and eosinophils were integrated into one immunological effect, namely the schistosomulicidal action.