The ability of Lactobacillus gasseri, a dairy lactic acid bacterium, to induce interferon (IFN) was investigated in murine macrophage cultures. IFN alpha was substantially induced by some strains of L. gasseri and the titers were the highest at a concentration of 100 micrograms ml-1 of L. gasseri DSM20243T. The expression of mRNA encoding IFN alpha was detected in spleen-macrophages (SP-M phi) and Peyer's patch-adherent cells stimulated with L. gasseri DSM20243T. Actinomycin D and cycloheximide added to SP-M phi cultures showed that the mRNA was synthesized by 0.5 h, and that IFN alpha was produced within 3 to 6 h after the stimulation with L. gasseri DSM20243T. The results support the notion that dairy products containing L. gasseri can be 'physiologically functional foods'.