The role and interdependence of CD8+ and CD4+ αβ-T cells in the acute response after respiratory infection with the murine parainfluenza type 1 virus, Sendai virus, has been analyzed for H-2b mice. Enrichment of CD8+ virus-specific CTL effectors in the lungs of immunologically intact C57BL/6 animals coincided with the clearance of the virus from this site by day 10 after infection. Removal of the CD4+ T cells by in vivo mAb treatment did not affect appreciably either the recruitment of CD8+ T cells to the infected lung, or their development into virus-specific cytotoxic effectors. In contrast, depletion of the CD8+ subset delayed virus clearance, although most mice survived the infection. Transgenic H-2b F3 mice homozygous (-/-) for a β2 microglobulin (β2-m) gene disruption, which lack both class I MHC glycoproteins and mature CD8+ αβ-T cells, showed a comparable, delayed clearance of Sendai virus from the lung. Virus-specific, class II MHC- restricted CTL were demonstrated in both freshly isolated bronchoalveolar lavage populations and cultured lymph node and spleen tissue from the β2-m (-/-) transgenics. Treatment of the β2-m (-/-) mice with the mAb to CD4 led to delayed virus clearance and death, which was also the case for normal mice that were depleted simultaneously of the CD4+ and CD8+ subsets. These results indicate that, although classical class I MHC-restricted CD8+ cytotoxic T cells normally play a dominant role in the recovery of mice acutely infected with Sendai virus, alternative mechanisms involving CD4+ T cells exist and can compensate, in time, for the loss of CD8+ T cell function.