PHA-stimulated lymphocyte supernatants (P-SUP) containing colony promoting activity (CPA) and interleukin-2 (IL-2) were found to drive T-lymphocyte precursor cells (OKT4-OKT8- blood cells) into colonies of mature T cells in agar. Ammonium sulphate precipitation followed by gel filtration on Sephadex G-75 showed that both IL-2 activity and CPA coeluted in the 21,000 MW fraction. Chromatofocusing on a PBE 94 gel equilibrated between pH 7.4 and 4.2 showed that CPA coeluted with the two main peaks of IL-2 (pI 7.2, pI 6.7). Adsorption of IL-2 activity by more than 97% on different T-cell lines did not completely abolish CPA. These results suggest that other(s) factor(s) with the same biochemical characteristics as IL-2 play a role in the induction of growth and differentiation of immature T-cell precursors.