Five monoclonal antibodies (mabs) specific for the envelope proteins of a simian immunodeficiency virus of African green monkeys (SIVagm) have been raised. Two mabs were directed against distinct epitopes on the transmembrane protein gp41. A conformational epitope on the gp130 was recognized by three mabs. This is the first report on mabs specific for SIVagm-gp130. Studies of the cross-reactivities revealed that the epitopes recognized by the env-directed mabs are conserved species-specifically in SIVagm isolates. Therefore, these mabs can be used to distinguish SIVagm strains from other virus groups.