Antibodies directed against the third hypervariable loop-domain (V3 loop) of human immunodeficiency virus type 1 (HIV-1) gp120 inhibit the infection by HIV-1 in a type-specific manner without interfering with the binding of gp120 to CD4. Previous studies demonstrated that soluble CD4 (sCD4) induced the dissociation of gp120 with gp41 and caused conformational changes within the envelope oligomer. We report changes in the binding and neutralizing activity of a monoclonal antibody against the V3 loop after sCD4 binding to gp120. Flow cytometry revealed that a type-specific neutralizing monoclonal antibody against V3 loop of HTLV-IIIB, 0.5 beta, reacted with HTLV-IIIMN-infected cells after exposure to sCD4. When the sCD4-treated HTLV-IIIMN infected cells were analyzed by two-color flow cytometry, most of the CD4-bearing cells were 0.5 beta-positive, indicating that this reactivity of 0.5 beta was associated with the binding of sCD4 to the infected cells. To determine the cross-neutralization by 0.5 beta after exposure to sCD4, HTLV-IIIMN viruses pretreated with sCD4 were used to infect susceptible target cells. The addition of 0.5 beta significantly reduced the p24 antigen production (66.1 +/- 5.9 pg/ml) compared with a control murine IgG (221.3 +/- 15.3 pg/ml). In contrast, no significant reduction in the p24 antigen production was observed by adding the HTLV-IIIMN neutralizing monoclonal antibody, mu 5.5, (209.9 +/- 15.0 pg/ml). Taken together, these results suggest that sCD4/gp120 binding could induce conformational/antigenic changes within the V3 loop that result in the induction of cross-reactivity and cross-neutralizing activity of a type-specific monoclonal antibody.