The ability to grow a clone of the cell line, MLA144, which is a constitutive producer of interleukin 2 (IL-2), in serum-free medium permitted the study of the direct effect of various agents on cell growth and IL-2 production in a homogeneous population. Bovine serum albumin (BSA) at 4 mg/ml was optimal for cell growth and IL-2 production. Selenium at 10 ng/ml enhanced IL-2 production nearly twofold and lithium at 42 ng/ml also enhanced IL-2 production by nearly twofold. Neither compound at these levels altered cellular proliferation. Two other compounds, iron and zinc, known to be associated with cellular proliferation and/or immunoregulation did not alter IL-2 production. Catalase or horseradish peroxidase was able to substitute for BSA and maintain the long-term growth of the MLA144 clone with only a 30% decrease in the rate of cellular proliferation and a 50% decrease in IL-2 production compared to cells maintained in the serum-free formulation with BSA. Addition of 0.5 mg of BSA to the catalase serum-free formulation increased the production of IL-2 to 70% of that of cells cultured in the BSA-containing serum-free formulation. The catalase-containing serum-free formulation has the advantage of consisting of only three proteins, catalase, insulin, and transferrin, at a very low protein content. The catalase-containing serum-free medium also supported the long-term growth of a human T-cell line, HSB-2.