The ability of antibody induced by MN and IIIB recombinant gp120 (rgp120) human immunodeficiency virus type 1 (HIV-1) vaccines to bind to oligomeric native and monomeric recombinant HIV-1 envelope glycoproteins (rgp120) was measured in 25 uninfected, healthy adult volunteers. A major focus was to evaluate the effect of simultaneous and sequential immunization with vaccines representing different strains of HIV-1 on the ability to broaden cross-reactivity of antibodies against these and other HIV-1 strains. A flow cytometric indirect immunofluorescence assay (FIFA) to detect vaccine-induced antibody to envelope glycoprotein expressed by infected and rgp120-coated target cells was used. MN rgp120 HIV-1 vaccine given alone and coadministered with IIIB rgp120 HIV-1 vaccine elicited antibody which bound to cells infected with HIV-1(MN), HIV-1(IIIB), HIV- 1(BF), and HIV-1(SF2). The presence of envelope glycoprotein-binding antibody detected by FIFA correlated to a moderate degree with functional antibody against HIV-I(MN) and HIV-1(IIIB). Priming immunization with IIIB rgp120 HIV-1 vaccine followed by booster injections of MN rgp120 HIV-1 vaccine resulted in increased cross-reactive antibody binding to these and heterologous clade B HIV-1 strains infecting cells. MN rgp120 HIV-1 vaccine given alone was better able to induce cross-reactive antibody to cells infected with heterologous HIV-1 laboratory strains than was IIIB rgp120 HIV-1 vaccine given alone. The vaccines induced binding antibody to rgp120 possessing the amino acid sequence of a clade E HIV-1 strain as measured by enzyme-linked immunosorbent assay. Levels of antibody binding to cells infected with clade B HIV-1 and cells coated with monomeric rgp120 were greater than that induced by HIV-1(IIIB)-based gp160 vaccines in previous studies.