Fungal β-glucans have abilities to induce NO (nitric oxide) synthesis by macrophages in vivo, and the intensity of NO synthesis significantly varied dependent on the structure of β-glucans. The molecular mechanism of NO synthesis by β-glucans, however, has not been clarified in detail. To determine β-glucan-mediated NO production, we used various β-glucans (SPG-OH, GRN; Grifolan, SSG, OL-2, ZYM; zymosan A and ZYC; zymocel), which could enhance NO synthesis in vivo, and stimulated peritoneal macrophages (PMs) in vitro in the presence of interferon-γ (IEN-γ). Using recombinant cytokines, a minimum concentration of the cytokines for NO induction was about 20 ng/ml in the presence of IFN-γ under the experimental conditions. Of β-glucans tested, only SPG-OH and GRN produced high concentrations of IL-6 in the culture supernatants. SSG also induced NO synthesis in vitro, but concentrations of inflammatory cytokines were low even in the presence of IFN-γ. In addition, there are some β-glucans which could induce NO synthesis in vivo but not in vitro (OL-2, ZYM, ZYC). These findings suggested that NO productivity of β-glucans in vivo is regulated by several mechanisms.