In this study, the prevalence of mycoplasmas in peripheral blood mononuclear cells (PBMC) from HIV-infected individuals was investigated using a mycoplasma genus-specific PCR assay. No mycoplasmas were detected in the PBMC samples from any of the 25 HIV-infected individuals (CDC 2, n = 8; CDC 3, n = 2; CDC 4, n = 15) or ten HIV-seronegative controls. As an internal control, HIV specific sequences were detected in the samples from all HIV-seropositives. These negative results do not support a suggested role of mycoplasmas as co-factor in the progression of HIV infection towards AIDS.