Tree nuts, including walnuts, are important elicitors of food allergy. We examined the ability of walnut kernel polyphenolics and purified ellagic acid (EA) to modulate cytokine production and cellular proliferation from stimulated human peripheral blood mononuclear cells (PBMC). IL-13 and TNF-α production decreased while no change was observed in IL-4 production. Paradoxically, EA and the walnut polyphenolics all significantly and dose-dependently inhibited stimulated [phytohemagglutin (PHA), α-CD3, and phorbol myristate acetate (PMA)/ionomycin] PBMC proliferation while simultaneously increasing IL-2 production. When added at time 0 min and 2 h, EA dose-dependently inhibited PHA-induced proliferation. However, at 30 min and 1 h, low doses of EA (10 and 1 μM) significantly increased proliferation above that of PHA alone, although higher doses led to inhibition. Our data do not support the hypothesis that walnut polyphenolics skew a cytokine response toward Th2 in an in vitro environment. However, immunomodulatory effects are present, including an inhibition of cellular proliferation despite no decrease in IL-4 or IL-2.