We investigated the occurrence of the CCR5Delta32 mutation in various regional ethnic groups in Brazil and tested the resistance of mutant peripheral blood mononuclear cells (PBMCs) to infection by HIV-1 in vitro. The heterozygous prevalence was 5.3% in uninfected African descendents and 8.8% in HIV-1-positive individuals (neither population had Delta32/Delta32). German descendents were 11% heterozygous and l% Delta32/Delta32. Amerindians were exclusively CCR5/CCR5. Heterozygous uninfected PBMCs showed partial resistance to R5-HIV-1 strains in vitro, but no resistance to X4 virus. HIV-1-positive CCR5/CCR5 had higher viral loads than did heterozygous cells.