Forty-six overlapping peptides (20-mers) representing the amino acid sequence of the external envelope glycoprotein of simian immunodeficiency virus (SIVmac; 32H isolate) were used to investigate linear antigenic sites recognized by antibodies in sera from SIV-infected rhesus macaques and in animals vaccinated with formalin-inactivated SIV. The reactivity to a discontinuous antigenic site as defined by a neutralizing monoclonal antibody was measured by competition assay. The majority of infected macaques recognized three linear antigenic determinants within the V1, V3 and C5 regions of the external glycoprotein. Animals infected with virus derived from the molecular clone SIVmac 32H (pJ5) showed broader reactivity to peptides with half of these animals having antibodies to the V2 region in addition to the V1, V3 and C5 regions. The majority of animals produced antibodies in response to the discontinuous epitope although these responses were weaker in animals infected with molecularly cloned virus. Seven of eight animals given vaccine in syntex adjuvant formulation (saf-1) produced antibodies in response to the discontinuous epitope and all reacted with peptides from the V1, V2 and V3 regions but only half recognized the C5 region. Animals receiving vaccine in alum adjuvant generally showed weaker responses to the discontinuous and linear determinants than those receiving saf-1 adjuvanted vaccine.