Mast cells from normal rats and animals reinfected with Nippostrongylus brasiliensis (N.b.) were sensitized in vitro with monoclonal anti-DNP mouse IgE, reaginic mouse serum with ovalbumin (OA) specificity and reaginic rat serum against N.b. The sensitized cells were triggered for the release of histamine with DNP-bovine-serum-albumin (DNP-BSA), OA, N.b.-homogenate and guinea-pig anti-mouse IgE. The histamine release from normal mast cells sensitized with monoclonal mouse IgE was inhibited either with N.b.-reaginic rat serum or OA-reaginic mouse IgE (30 micrograms/10(5) mast cells) completely desensitized mast cells from reinfected rats for specific histamine release in the presence of either N.b.-antigen or DNP-BSA. Anti-mouse IgE which was prepared by monoclonal mouse IgE did not bind to rat mast cells sensitized with rat IgE as was revealed by immunofluorescence experiments. Consequently we observed that anti-mouse IgE failed to trigger the histamine release from mast cells of reinfected rats.