The antibody recognition of the major neutralization epitopes of human immunodeficiency virus type 1 (HIV-1) in 829 HIV-1-seropositive subjects from North America (106), Europe (241), Africa (342), and Asia (100) was investigated. Peptides derived from diverse published V3 loop sequences were used as antigen, and serum reactivity was detected by sensitive ELISAs. Antibody binding to peptides derived from the V3 loop sequence of HIV-1 isolates varies considerably depending on the geographic origin of the antibody and is associated with neutralization titer against homologous isolates. Serotype reactivity to peptides may be a simple and rapid approach to investigation of HIV-1 env diversity worldwide and may assist the choice of immunogen for development of future AIDS vaccines.