A fibroblastic cell line GCG derived from grass xarp gill was established by using 199 medium plus 10% feral calf serum at 25℃. The cell lime had three exponential growth phases over an incubation period of 12 days with the doubling time of 51 hours in the first phase. Its chromosomal model number was 46, minimal plating dense, 15,000 cells/ml, and cloning efficency, 3%. 10% fetal calf serum was required for its normal growth. The cell line was sensitive to grass carp rhabdvirus. It has been subcultured for more than 90 passages since its initiation in November 1987. Based on the unique properties of the growth curve and additional data, a hypothesis concerning the control of the division of cells in vitro was ptoposed. It was advaneed that (1)cell division was controled by a pair of contraacting cellular factors-Mitosis Inhibition Factor(MIF) and Miosis Relase Factor(MRF); (2) MIF inhibited cell division at a certian concetration threshold and was monitored by feedback control; (3) the quantities of MIF and MRF that a given cell line synthesized were characteristic of the cell. The growth behavior of normal and malignant cells in vitro were explained by using this theory.