Human immunodeficiency virus type 1 (HIV-1) displays both interstrain and intrastrain genetic variability. Virus populations with extensive microheterogeneity have been defined as swarms or quasispecies. Many of the genomes within HIV-1 swarms appear to be defective in one or more genes required for viral replication. It is unclear to what extent defective viruses play a role in the process of HIV-1 infection or in the pathogenesis of AIDS. We have isolated two biologically active HIV-1 clones: LW 12.3, which contains defects in the vif and vpr genes, and MN ST.1, which has a defect in the vpu gene. LW 12.3 is unable to replicate in peripheral blood mononuclear cells (PBMC). The growth of MN-ST.1 in SupT1 cells is marked by a 3-week lag in extracellular virus production and by the presence of unusually abundant viral buds. We demonstrate here that coinfection of PBMC with these two partially defective HIV-1 clones extends the cellular host range of LW 12.3, significantly increases the replication rate of both viral genomes, and eliminates the delay in production observed with the vpu-defective MN ST.1. When the lesions in vpr and vif of LW 12.3 are repaired, the resultant virus grows normally in PBMC. This is also the case when only vif is repaired, indicating that complementation of LW 12.3 in PBMC by MN ST.1 is mediated by vif in trans. The reciprocal complementation results in a dramatic increase of HIV-1 virulence. This two-component model represents a simplified version of the in vivo situation and illustrates one way in which interaction of defective viruses could increase the spread of infection and progression of disease.