It has first been established that interferon preparations, especially reaferon, exert a protective antiviral effect in nonepidemic situations under conditions of physical and emotional stress leading to activation of opportunistic flora. Alongside with antiviral effect, reaferon showed a significant immunomodulating effect on the host reflected in blast transformation of T-lymphocytes and production of gamma-interferon. The immunoregulating action of reaferon was prolonged. These data suggest that exogenous use of reaferon may be one of the methods enhancing the resistance to emotional and physical stress.