SIVmac infection of macaques is an important animal model for HIV infection and AIDS; this model is being utilized for development of antiviral therapies and vaccines. In the present article, we sought to identify neutralization epitopes of SIVmac envelope surface glycoprotein (gp130). Algorithms were used to predict antigenicity of specific regions. Four regions from the primary amino acid sequence of the viral surface glycoprotein were selected. A synthetic peptide representing one of these regions (414-434) induced virus-neutralizing antibodies in mice; in addition, this peptide induced T cell-proliferative responses in macaques. To address the in vivo relevance of these observations, we demonstrated that experimentally infected macaques produce antibodies to the neutralization epitope. In addition, rhesus macaques protected against infection by an inactivated SIV vaccine develop antibodies that bind to peptide 414-434. These observations demonstrate that the region that includes the sequence 414-434 in the fourth variable domain (V4) of SIVmac gp130 contains both a linear neutralization epitope and a T cell epitope.