The envelope gene, especially the V(3) region, of HIV-1 has been shown to be a principal determinant of cell tropism, replication and cytopathogenicity of the virus. In addition, the V(1)/V(2) region of the envelope gene has been found to be an important factor in cell tropism. We examined the compatibility between the V(1)/V(2) and C(2)-V(3) domains of HIV-1 gp120 in different combinations on viral replication by using envelope recombinants between ME1 and ME46, two infectious molecular clones with diverse biologic activity longitudinally isolated from one seropositive subject. Our data demonstrate that a proper interaction between the regions of V(1)/V(2)and C(2) is essential for viral infection and hence replication. Sequence analysis and subsequent site directed mutagenesis study indicate that the pattern of potential envelope N-glycosylation in the V(1)/V(2) and C(2)-V(3) regions may be the determining factor in such interaction between these two regions. It is possible that improper N-glycosylation sites while not affecting virus assembly, can influence through steric hindrance the conformational change of the V(3) region that is required for the co-receptor attachment and hence the viral infectivity.