In vivo passage of a simian-human immunodeficiency virus (SHIV-89.6) generated a virus, SHIV-89.6P, that exhibited increased resistance to some neutralizing antibodies (G. B. Karlsson et al., J. Exp. Med. 188:1159–1171, 1998). Here we examine the range of human immunodeficiency virus type 1 (HIV-1) neutralizing antibodies to which the passaged virus became resistant and identify envelope glycoprotein determinants of antibody resistance. Compared with the envelope glycoproteins derived from the parental SHIV-89.6, the envelope glycoproteins of the passaged virus were resistant to antibodies directed against the gp120 V3 variable loop and the CD4 binding site. By contrast, both viral envelope glycoproteins were equally sensitive to neutralization by two antibodies, 2G12 and 2F5, that recognize poorly immunogenic structures on gp120 and gp41, respectively. Changes in the V2 and V3 variable loops of gp120 were necessary and sufficient for full resistance to the IgG1b12 antibody, which is directed against the CD4 binding site. Changes in the V3 loop specified complete resistance to a V3 loop-directed antibody, while changes in the V1/V2 loops conferred partial resistance to this antibody. The epitopes of the neutralizing antibodies were not disrupted by the resistance-associated changes. These results indicate that in vivo selection occurs for HIV-1 envelope glycoproteins with variable loop conformations that restrict the access of antibodies to immunogenic neutralization epitopes.