The injection of Trichina extract into guinea-pig footpads led to eosinophilia in the draining lymph nodes. Primary injections resulted in para-follicular accumulations within 24 hr in about 15% of animals tested; multiple injections resulted in diffuse eosinophilia in all animals, and, after it was allowed to dissipate, a brisk anamnestic response was elicited by a single challenge. Diethylcarbamazine eliminated the primary response, but had no effect on the secondary response. The drug was effective only when given (5 min–3 hr) before the extract; it failed to block when given (5 min or 1 hr) after the extract. The data indicate that the eosinophilia which accompanies Trichina infestation is, at least in part, an immunologic response to larval antigens, and that SRS-A may be one of the mediators.