OK432 (preparation derived from the Su strain of Streptococcus pyogenes A3; Picibanil, CAS 39325-01-4) is an immunomodulator. The treatment of mice with OK432 enhances their resistance to encephalomyocarditis virus (EMCV) along with a concomitant increase of interferon (IFN) titer and natural killer (NK) cell activity. To ascertain whether IFN or NK cell activity may play a crucial role in the mechanism of resistance, we compared these strains: EMCV resistant C57BL mice, C3H mice with myocarditis and DBA/2 mice with both myocarditis and diabetes mellitus. Although IFN production in all three kinds of mice was significantly increased on day 3 after inoculation, NK cell activity in EMCV resistant C57BL mice was significantly lower than that in C3H and DBA/2 mice. The lower antiviral resistance of mice treated with both OK432 and anti-interferon antibody (aIFN) was accompanied by a reduction of serum IFN titer, irrespective of the reduction in NK cell activity. Decreased activation of NK cells by anti-asialo GM1 monoclonal antibody (aNK) of OK432-treated mice also resulted in higher viral titers. However, these titers of both OK432 and aNK-treated mice were significantly lower than those of both OK432 and aIFN-treated mice. The degree of elevation of viral titer showed the following trend: OK432 and a IFN-treated mice > OK432 and aNK-treated mice >> OK432-treated mice. Moreover, histological changes of the heart in OK432 and aIFN-treated mice were significantly (p < 0.05) more severe than that in OK432 and aNK-treated and that in OK432-treated infected mice 7 days after inoculation.(ABSTRACT TRUNCATED AT 250 WORDS)