In previous studies (1–4), we described disrupted hematopoiesis in long‐term human bone marrow cultures infected with HIV. Conditioned medium (CM) from HIV‐1 non‐productively infected liquid cultures inhibits the proliferation of granulo‐monocytic progenitor cells (CFU‐GM) in clonogenic assays. A growth‐inhibitory soluble factor was therefore suspected and we report here that this factor has been identified as the nef gene product. These nef‐containing supernatants are specifically neutralized by anti‐nef antibodies and recombinant nef has the same growth inhibitory properties. We used a nef‐deficient virus to confirm that nef protein is responsible for the in vitro growth inhibition of CFU‐GM.