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To understand the relationship between growth factor-induced mitogenesis and spontaneous cell transformation, a clonal isolate of epidermal growth factor (EGF)-responsive NRK cells was passed in vitro until morphologically transformed variants arose. Subclones of EGF responsive (Cl-3) and EGF nonresponsive (Cl-10) NRK cells were isolated. Cl-3 cells grew as flat, contact-inhibited monolayers, while Cl-10 cells grew as rounded or spindle-shaped cells that formed dense foci. Cl-10 cells formed colonies in soft agar more efficiently (p less than 0.01) and formed larger tumors in nude mice (p less than 0.05) than Cl-3 cells. Cl-3 cells exhibited a sixfold increase in DNA synthesis in response to 1.0 nM EGF. Cl-10 cells did not increase DNA synthesis on exposure to 100 nM EGF. These different responses to EGF occurred despite similar numbers of receptors and similar receptor.binding affinities for EGF (Cl-3: 7000 receptors, Kd = 0.67 nM; Cl-10: 8000 receptors, Kd = 0.72 nM). No evidence of transforming growth factor-alpha was detected in either of these cell lines using Northern blots, Western blots, or biologic assays. We conclude that NRK cells which undergo spontaneous morphologic transformation and exhibit enhanced anchorage-independent growth lose their mitogenic response to EGF.