A novel experimental approach was employed to investigate the protective role of cellular and humoral factors in antiviral immunity under conditions of their prophylactic administration into the tracheobronchial cavities of normal mice-recipients which were challenged 24 hr later with influenza virus A/PR/8/34 (H0N1). The highest protective effect was afforded by intranasal administration of immune serum (91.3%) as compared with splenocytes (65.2%). The addition of immune serum to cell suspensions or pooled lymphocyte and macrophage fractions previously isolated from the spleen increased the protective effect of these materials by 97--100% thus surpassing the protective effect of immune serum alone. The protective effect of passively administered cells from inflammatory exudates of peritoneal or tracheobronchial cavities was higher than that of a more concentrated splenocyte suspension; the results were comparable only if the number of splenocytes was 4- to 5-fold that of the exudate cells. The degree of the protective effect was strictly proportional to the concentration of immune system cells administered to the concentration of immune system cells administered to the syngeneic recipients.