Abstract We investigated whether reduction of the phytohemagglutinin (PHA)-induced proliferative response of lymphocytes from HIV type-1 (HIV-1)-infected (HIV +) individuals could be explained by overproduction of transforming growth factor-β1 (TGF-β1), a strong inhibitor of T cell proliferation. PHA-stimulated PBMC from 40 HIV - and 42 HIV + homosexual men from the Baltimore Center of the Multicenter AIDS Cohort Study (MACS) were studied using Northern blot analysis of expression of TGF-β1 mRNA and determining the effects of anti-TGF-β1 neutralizing antibody on PHA-induced proliferative responses. Compared to the HIV - donors, HIV + donors did not show increased expression of TGF-β1 mRNA in unstimulated or PHA-stimulated PBMC. Furthermore, a neutralizing antibody to TGF-β1 did not reverse the decreased proliferative response of PBMC from HIV + individuals to PHA or interleukin-2. These results indicate that TGF-β1 is not involved in T cell proliferation defects seen in HIV + donors.