Polyethylene glycol was used to induce polykaryon formation among U-937 cell subclones carrying defective human immunodeficiency virus (HIV) type 1 proviral DNA. Fusion of cells which produced gp120-defective virions (UHC15.7) with cells unable to generate reverse transcriptase (RT) activity (UHC8 and UHC18) yielded polykaryons which made infectious viral progeny that showed normal protein profiles. Southern blot analysis of proviral DNA of cells infected with such fusion-derived virus revealed a restriction map identical to that of cells harboring infectious parental-type HIV type 1 (U-937/UHC1). These results suggest that repair mechanisms involving genetic recombination(s) play a role in the generation of infectious virus after fusion of cells which harbor defective HIV.