In this study, we compared sera from 159 human immunodeficiency virus type 1 (HIV-1)-infected individuals from Tanzania and 103 infected individuals from the United States for antibodies reactive with 10 HIV-1 gp160 epitopes defined by synthetic peptides. Our data indicate that the anti-gp160 antibody fine specificity differs between infected individuals from these two geographically diverse populations. For example, 50% of the Tanzanian sera contained antibodies reactive with an immunodominant HIV-1 gp41 epitope defined by peptide 600-611, whereas 91% of the sera from the United States were reactive. Differences in serologic reactivity between HIV-1-infected individuals from Tanzania and the United States were also observed with gp160 epitopes defined by peptides 503-528 and 846-860. Included among the peptides examined were four which corresponded to the V3 region of gp120. The majority of sera from either country contained antibodies reactive with peptide RP142, whose V3 sequence is based upon that of HIV-1 isolate MN. Further characterization of serologic reactivity suggested that sera from Tanzania were more likely to neutralize HIV-1 isolate IIIB or MN in vitro than were sera from the United States. These differences in antibody fine specificity between HIV-1-infected individuals from Tanzania and the United States suggest that regional isolates of HIV-1 may exist.