Human epithelioid cells (NHIK 3025) are able to traverse at least one cell cycle, if the serum concentration in the medium is reduced to only 0.3% when the cells are in mitosis. The cell cycle and protein doubling times were prolonged considerably under such conditions. The protein doubling time was, however, prolonged much more than the cell cycle. The addition of purified growth factors (insulin, MSA, EGF, FGF or PDGF) did not shorten G1 or the cell cycle, although insulin, EGF and PDGF increased the rate of protein synthesis. When the growth factors were added to the medium two and two in combination, it was found that four combinations (out of ten possible) increased the rate of progress through G1, whereas three combinations increased the rate of progress through the total cell cycle. Only two combinations increased the rate of protein accumulation to such an extent that the protein content doubled during the cell cycle. Both of these combinations also shortened the cell cycle. Since different combinations of growth factors were found to regulate the various events related to growth of NHIK 3025 cells, it is suggested that these events are under separate control.