Exogenous growth factors normally required in cell culture activate cell proliferation via the mol. controls of cell-cycle progression. Highly differing influences of mitogenic stimulation of Chinese hamster ovary (CHO) cells by insulin and basic fibroblast growth factor (bFGF) have been clearly obsd. in a defined protein-free medium. CHO K1 cells stimulated only with insulin grow with flattened cell morphol. and extensive cell-cell contact, whereas stimulation with only bFGF or bFGF plus insulin results in loss of cell-cell contact and a transformed and rounded-up morphol. Compared with insulin-stimulated cells, bFGF-stimulated cells exhibit a relatively long G1 and short S phase, and contain higher levels of cyclin E. Observation of elevated levels of cyclin E in wild-type CHO K1 cells mitogenically stimulated by basic fibroblast growth factor motivated transfection of these cells by a cyclin E expression vector. These transfectants grew rapidly in protein-free basal medium and had similar cyclin E levels, distributions of nuclear cell-cycle times, and cell morphologies as bFGF-stimulated CHO K1 culture. Metabolic engineering of cell-cycle regulation thus bypasses exogenous growth factor requirements, addressing a priority objective in economical, reproducible, and safe biopharmaceutical manufg. [on SciFinder (R)]