A two-stage tumor growth model that explicitly incorporates mitotic rate of initiated cells is developed. As an example, the model is applied to data from dinitrotoluene (DNT) studies to demonstrate how information on preneoplastic and neoplastic lesions can be incorporated into the dose-response model, to critically examine its implication for quantitative risk assessment, and to simulate issues, problems, and data gaps that may arise when one attempts to incorporate cell proliferation into the quantitative risk estimation.