Swiss 3T3 fibroblasts can be weakly stimulated to grow by bombesin, epidermal growth factor or ceruloplasmin when cells are maintained in Dulbecco's Modified Essential Medium (DMEM), the pH of which is 7.75. Addition of insulin synergizes with the other mitogens. However, only ceruloplasmin promotes DNA synthesis in Minimum Essential Medium (MEM). The pH in this medium is 7.0. All the other growth factors synergize with the ceruloplasmin effects, but such synergism is not evident with insulin. If the pH in MEM is increased to 7.25 or 7.75 by supplementation with HEPES or NaHCO3, respectively, the results are similar to those found in DMEM. Since the oxidation of iron is increased at alkaline pH, the reoxidation of iron at the cell surface may facilitate growth at alkaline pH. We propose that iron reoxidation is limiting for cell growth and that part of the ceruloplasmin effect is mediated by its action as a terminal oxidase for ferrous iron on the cell surface. Observations consistent with this explanation include: 1) combinations of insulin with bombesin or epidermal growth factors do not promote cell proliferation at pH 7.0; 2) fetal calf serum, which has ferroxidase activity, and ceruloplasmin plus or minus other growth factors stimulate cell proliferation at pH 7.0; and 3) alkaline pH also restores the mitogenic effect of growth factors.