The distribution of phenotypes of the group specific component (Gc) was examined in 203 homosexuals at risk of infection or infected by the human immunodeficiency virus and compared with that in 50 randomly selected homosexuals and 122 healthy male heterosexual seronegative controls. 30.2% of patients with the acquired immunodeficiency syndrome (AIDS) were homozygous for Gc 1 fast (Gc 1f) compared with 0.8% of controls (p less than 0.0001); patients with other clinical manifestations of HIV infection were also more likely than controls to have Gc 1f. By contrast, seronegative symptomless homosexual contacts of AIDS patients (AH-p) lacked this phenotype but were more likely than controls to be homozygous for Gc 2 (25% vs 9%, p less than 0.05). AIDS patients lacked the homozygous Gc 2 phenotype altogether. A chi 2 trend test showed that progression to AIDS had a strong positive association with the Gc 1f allele (p less than 0.0001) and a negative one with Gc 2 (p less than 0.05). It is proposed that Gc may be involved in viral entry into host cells, the ease of which varies with different allelic forms of Gc, according to their sialic acid content.