The infection of human cells by laboratory strains of human immunodeficiency virus type 1 (HIV-1) can be blocked readily in vitro by recombinant soluble CD4 and CD4-immunoglobulin hybrid molecules. In contrast, infection by primary isolates of HIV-1 is much less sensitive to blocking in vitro by soluble CD4-based molecules. To investigate the molecular basis for this difference between HIV-1 strains, we isolated the gp120-encoding genes from several CD4-resistant and CD4-sensitive HIV-1 strains and characterized the CD4-binding properties of their recombinant gp120 (rgp120) products. Extensive amino acid sequence variation was found between the gp120 genes of CD4-resistant and CD4-sensitive HIV-1 isolates. However, the CD4-binding affinities of rgp120 from strains with markedly different CD4 sensitivities were essentially the same, and only small differences were observed in the kinetics of CD4 binding. These results suggest that the lower sensitivity of primary HIV-1 isolates to neutralization by CD4-based molecules is not due to lower binding affinity between soluble CD4 and free gp120.