We screened for foodstuffs affecting interferon-beta (IFN-beta) production of human foreskin diploid fibroblasts, in the presence of poly I.poly C, cycloheximide, and actinomycin D. alpha- and beta-caseins stimulated IFN-beta production dose-dependently, but kappa-casein inhibited it. Of the two chymosin fragments of kappa-casein, glycomacropeptide was an inhibitor but p-kappa-casein was not. beta-lactoglobulin stimulated IFN-beta production weakly, but lactoferrin inhibited it strongly. It was also shown that serum contained some factors inhibiting IFN-beta induction and stimulating its production.