Intravenous inoculation of Trichinella larvae into rats is followed by eosinophilia with a peak at 6 days. Reinjection of the parasites 20 days later results in an enhanced eosinophil response. This phenomenon may be analogous to the anamnestic antibody response which follows reinjection of antigenic material. Various agents known to modify conventional expressions of immunity were tested for their abilities to influence both primary and secondary eosinophil responses. Given in low single doses, methotrexate and cyclophosphamide could suppress the eosinophil response only after larval challenge; busulfan, prednisolone, bacterial endotoxin, Corynebacterium anaerobium and pertussis vaccine were effective only before the inoculum of parasites. The time-dependent action of these agents on eosinophilia is similar to their action on other systems of immune expression. Of further interest was the observation that certain of the agents which were suppressant when given before first larval challenge also appeared to affect "priming" for the second response. Eosinophilia appears to share common features with recognized immune phenomena. The model described may be useful in studying the cellular kinetics of immune responses, the action of immunosuppressive agents and other problems of cell differentiation or proliferation.