Previous studies in our laboratory suggested that Lactobacillus acidophilus strain DDS-1 (LA1) has a suppressive effect on chemically induced tumors in experimental animals. In an effort to understand the possible mechanisms underlying this effect, we investigated the ability of LA1 to induce the production of interleukin-1 alpha (IL-1 alpha) and tumor necrosis factor-alpha (TNF-alpha), which have potent cytocidal and cytostatic effects on tumor cells. The mouse macrophage cell line RAW264.7 was incubated with live or heat-killed cells of four strains of L. acidophilus or Bifidobacterium bifidum. Escherichia coli was used as a source of lipopolysaccharide that is known to induce the above cytokines. The amount of the cytokines present in the culture fluid was quantitated by an enzyme-linked immunosorbent assay. LA1 induced the production of higher levels of IL-1 alpha and TNF-alpha than other lactobacilli and bifidobacteria. Stimulation of the production of the cytokines was not due to the lipopolysaccharide (LPS) component, since LPS at concentrations equivalent to, or 100-fold greater than, that of LA1 induced only negligible amounts of IL-1 alpha and TNF-alpha. These results reveal that non-LPS component(s) of LA1 stimulate(s) the production of IL-1 alpha and TNF-alpha by macrophages, indicating that this organism stimulates the production of immunologic factors.