The genetic polymorphism of group-specific component (GC) was investigated with isoelectric focusing in 351 homosexual men at risk for HIV infection, 96 male patients with AIDS, and 86 heterosexual controls. No significant differences in GC phenotype distribution were seen between controls and any of the at risk groups or patients, neither between HIV-Ab-positive/Ag-negative and HIV-Ab-positive/Ag-positive homosexual men nor between HIV-Ab-positive/Ag-positive homosexual men and AIDS patients, suggesting that the GC system is not involved in the infective susceptibility or progression of HIV infection to AIDS-related complex and AIDS.