P3C and LPS act in synergy with antigen to markedly potentiate production of cytokines in MC/9 cells. IgE-primed MC/9 cells were stimulated in complete growth medium with the indicated concentrations of antigen (Ag), P3C, and LPS, individually or in the combinations indicated. Levels of TNFα (A-C), IL-6 (D), and IL-13 (E,F) were determined by ELISA 3 hours (A-E) or 6 hours (F) after addition of stimulants. Vehicle was substituted Ag (A,C,D,E, and F) or PC3 (B) where the concentration is designated as 0. Values are the mean ± SEM from 3 cultures. Asterisks indicate significant increase in response as compared to the responses to antigen (*P < .05; **P < .001). Identical results were obtained in 2 other experiments.