Experiments in 400 non-inbred white mice indicated that the association of influenza virus A2 Hong Kong with H. influenzae enhanced the pathogenic action of the causative agents and led to the unfavorable clinical outcome only in those cases when these agents penetrated the body simultaneously, or when viral infection preceded bacterial infection. In those cases when influenza infection appeared in the presence of bacterial infection, the pathological process developed as a monoviral disease. The morphological changes in the lungs of the mice infected with H. influenzae corresponded to lesions caused by influenza virus and were manifested by pronounced hemodynamic disturbances.