125I-Insulin was used as a model mitogen to examine the relationships of mitogen receptor occupancy and exposure in controlling cell replication. Labeled hormone was bound to substratum-attached, confluent fibroblasts with two affinities, K1 approximately equal to 2 X 108 M-1 and K2 approximately equal to 0.8 X 107 M-1. Approximately 9,000 receptors per cell were calculated from a scatchboard plot analysis with 80% of them of the affinity type. Treatment of confluent fibroblasts with 1.0 to 10 microng/ml of trypsin for 10 min increasd the number of exposed K2 sites by a factor of 2 to 3. Culturing the fibroblasts in the absence of serum for 12 to 24 hr also increased the quantity of exposed K2 sites to 60,000 per cell. The addition of unlabeled insulin to untreated, trypsin-treated, or serum-starved fibroblasts resulted in a stimulation of 2-deoxyglucose transport and thymidine incorporation activity that was proportional to the observed level of 125I-insulin binding. The quantity of exposed insulin receptors on uninfected fibroblasts decreases as the cell culture density increases. Hormone binding to B77 virus-transformed fibroblasts also decreases with culture density but plateaus at a density of 6 X 105 cells/plate. This resulted in a 2- to 3-fold difference in the level of exposed receptors between the uninfected and virus-transformed cells in confluent cultures, and proportionally higher rates of sugar transport and thymidine incorporation activity. Trypsin treatment and 12 hr of growth in the absence of serum did not result in an increase in the level of recepto exposure in the virus-transformed cells. The results of this study suggest that the pleotypic events associated with the stimulation of cell replication as represented by sugar transport and DNA synthesis are dependent upon and proportional to mitogen receptor occupancy. The control of cell replication, however, appears to be linked by a presently unknown mechanism with the degree of exposure of mitogen recipotrs. Receptor concentration is high and is similar in rapidly growing uninfected and virus-transformed cells, but the subsequent decrease observed with an increase in culture density reaches an early plateau for the transformed cells. This difference in exposed receptors could provide the growth advantage that is the hallmark of transformed cells.