The kinetics of human immunodeficiency virus type 1 (HIV-1)-induced cell death were investigated in cell-to-cell and cell-free models of virus transmission. Cocultivation of HIV-1 chronically infected H9 donor cells with uninfected H9 recipient cells resulted in rapid induction of programmed cell death. Within 8 h, apoptotic chromatin condensation was identified by histologic staining. In addition, many single cells with apoptotic nuclei were observed, indicating that stable cell fusion was not a requirement for apoptosis to occur. By 12 to 18 h of coculture, a DNA fragmentation ladder characteristic of apoptosis was detected by agarose gel electrophoresis. Quantitation of apoptosis by measurement of nuclear DNA content revealed that at least 20 to 30% of the nuclei were undergoing apoptosis by 24 h after cocultivation. The appearance of condensed nuclei and fragmented DNA occurred as HIV reverse transcription was completed, and it was not inhibited by zidovudine, suggesting that induction of apoptosis did not require new HIV replication. Soluble CD4 inhibited apoptosis, demonstrating that Env-CD4 interactions were required for apoptosis. In contrast to that in cell-to-cell transmission, apoptosis in cell-free HIV infections was markedly inefficient and was not observed until 70 to 90 h after infections were initiated. These findings indicate that HIV-1 induction of programmed destruction of the nucleus is initiated at the time of cell-cell cocultivation by a mechanism which requires CD4-Env interactions but not new HIV replication.