We describe experiments comparing the proliferation responses to epidermal growth factor (EGF) by NR6 fibroblasts expressing genetically engineered epidermal growth factor receptors (EGFRs). These cells present either wild-type (WT) EGFR or a cytoplasmic domain-truncated (c’973) EGFR that exhibits a decreased ligand-induced internalization rate constant. In two distinct in vitro proliferation assays, with or without medium replenishment, we measured the specific cell proliferation rate constants and EGF depletion kinetics for both WT and c’973 cells. When EGF depletion is minimized by replenishment, the EGF concentration dependencies of the two cell types are similar, whereas when EGF depletion is not prevented, maximal proliferation of WT cells requires an initial EGF concentration that is approximately 10X that required by c’973 cells. However, when EGF depletion is accounted for, the dependencies of growth rate for the two cell types on the current EGF concentration in both assays are essentially identical. Our results demonstrate that diminished depletion of EGF from the extracellular medium is a major reason for increased mitogenic sensitivity to EGF by cells possessing internalization-deficient receptors.