The proteolytic cleavage sites of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein precursor gpl60 and the fusion protein of respiratory syncytial virus (RSV) show a sequence homology. To study this homology two synthetic peptides corresponding to HIV-1-env-gpl60-aa 507–518 (KAKRRWQREKR) and RSV-F2-aa 130–136 (SKKRKRR) were synthesized. Human serum samples from HIV-positive or RSV-positive collections recognized the appropriate peptide in 90.6 or 37.2% respectively. No cross-reactivity towards the non-homologous peptide could be monitored in both serum collections. In contrast, antipeptide antibodies raised against both peptides demonstrate a high degree of cross-reactivity. These data indicate that the high specificity of the virus-induced antibodies may be a result of strong conformational restrictions at the proteolytic cleavage site of both proteins. Moreover, these observations are important for diagnostic purposes. Synthetic peptides are a valuable tool for HIV antibody screening. Our data provide information concerning the specificity of antigen-antibody interaction on a highly immunogenic HIV-1 epit