We have studied antibody reactivity with monomeric and oligomeric forms of the gp120 envelope glycoprotein from the macrophage-tropic primary virus, HIV-1 JR-FL. We find that the correlation between oligomer reactivity and virus neutralization is not absolute for MAbs to epitopes overlapping the CD4-binding site on gp120. An MAb (205-46-9) with very limited neutralizing ability for JR-FL binds about as avidly to oligomeric JR-FL envelope glycoproteins as the strongly neutralizing IgG1b12 MAb does. In addition, neutralizing and nonneutralizing sera from HIV-1-infected people are similar in their reactivities to oligomeric JR-FL envelope glycoproteins; the correlation between oligomer reactivity and virus neutralization is weak. Although oligomer reactivity of an anti-gp120 antibody is necessary for virus neutralization, it is not always sufficient to cause it.