Human immunodeficiency virus type 1 (HIV-1) isolates display differences in a variety of in vitro biological properties, including the ability to infect different cell types, the kinetics of replication, and cytopathicity in the infected cells. Studies with isolates obtained from the same individual over time have shown that these in vitro properties of the viral isolates correlate with pathogenicity in the host. The later isolates, recovered when disease has developed, display a wider cellular host range, replicate rapidly and to high titers in the infected cells, and induce syncytia in these cells. In the present studies, the genomic determinants of these biological properties were defined with recombinant viruses generated between two HIV-1 isolates recovered sequentially from the same individual. The results show that the rate of HIV-1 replication in the HUT 78 T-cell line is controlled by the first coding exon of tat. Infection of T-cell and monocytic cell lines is determined by two specific regions in the envelope gp120, one of which also confers the ability of an isolate to induce syncytia. Amino acid sequence comparison of the regions identified revealed minor differences between the two viral isolates: 2 amino acids in the tat gene product and 10 and 12 amino acids in the two regions of envelope gp120. These data suggest that small changes in the tat and env proteins can have dramatic effects on the pathogenic potential of HIV-1.