Neutralizing antibodies (NAs) against four isolates of human immunodeficiency virus type 1 (HIV-1) were assayed in HIV-1 antibody positive sera from the United States, Haiti, Zimbabwe, and Zaire. Overall, there were NAs detected in 95, 81, 60, and 73% of sera with reciprocal geometric mean titers (GMTs) of 626, 23, 10, and 20, respectively, against HIV-1MN, HIV-1IIIB, HIV-1RF, and HIV-1Z3. Sera from North America had significantly higher NA titers against HIV-1MN. In each country, the highest antibody titers observed were against the MN strain. Otherwise, sera from the U.S. neutralized most strongly HIV-1IIIB, sera from Zaire neutralized most strongly HIV-1Z3, and sera from Zimbabwe had equal titers against all three viruses. The differences between countries were reflected in analyses of NA titers of subgroups classified on the basis of clinical status, indicating that the differences were not likely to be related to differences in clinical status of the patients being tested. Some of this antigenic variation is reflective of known genetic diversity, while some is not. The results suggest that undefined preserved and variable regions containing neutralization epitope(s) exist. These data do not indicate a need to define antigenic subtypes of HIV-1 at present. The existence of conserved neutralization epitope(s) may indicate the potential for broad immunogenicity of appropriately selected vaccine antigens.