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The Cytosensor Microphysiometer, a biosensor developed by Molecular Devices, was used to assay rapid binding activity of IGF-I for bovine mammary epithelial cell lines. Insulin-like growth factor-I (IGF-I) is a potent mitogen for mammary epithelial cells and has been implicated in breast cancer cell proliferation, a leading cause of cancer death of women in the U.S. today. IGF-I acts by binding to cell surface receptors. We are interested in how autocrine secretion might alter the activity and regulation of IGF-I. Real-time changes in excretion of protons, which we assert results from IGF-I binding, are detected by the Cytosensor Microphysiometer and can be correlated with cellular activity. We present IGF-I dose-dependent responses as well as correlated binding data to detect cell surface receptor concentration and thymidine incorporation results to determine cell proliferation following IGF-I stimulation. We examine the effect of insulin-like binding protein-3 (IGFBP-3) both in the presence and absence of IGF-I. We believe comparison of autocrine and paracrine environments for IGF-I stimulation, and the components contributing to the binding of IGF-I to the cell membrane receptor may provide pertinent information for the development of intervention schemes to slow or interrupt IGF-I binding to tumor cells and therefore cancer growth.