The biological properties of homogeneous populations of feline immunodeficiency viruses derived from infectious, molecular clones of the TM1, TM2 and Petaluma strains were compared. Differences in infectivity for Crandell feline kidney (CRFK) cells, and in syncytium formation and replication kinetics in a feline T lymphoblastoid cell line (MYA-1 cells) were observed. To investigate the basis of these differences between the TM2 and Petaluma strains, we first compared the basal promoter activity of the long terminal repeat which is a highly divergent region, but no significant difference in activities was found in CRFK cells. We then constructed two recombinant chimeric clones which carry gag, pol, vif, and ORF A from the heterologous virus. From analyses using the chimeric clones, it was revealed that efficient virus growth in CRFK cells and MYA-1 cells was regulated by the gag, pol, vif and ORF A regions, whereas viral determinants of infectivity for CRFK cells, and syncytium formation and cytopathogenicity in MYA-1 cells, were located in the env region.