The replication kinetics of HIV were examined in HTLV-I-transformed MT-2 cells. The duration of the initial replication cycle was 20 hours, determined by the first detection of infectious progeny virus, development of syncytia, and production of viral RNA and protein. A phase of exponential virus production followed until 62 h postinfection. Cell death occurred in the final phase of infection during which infectious virus production remained constant even though viral RNA and protein production increased at an exponential rate. Accumulations of HIV particles were observed within cytoplasmic vacuoles of infected MT-2 cells. Although cell lysates contained high titers of infectious virus, our data show that an increasing proportion of particles produced late in infection were not infectious.