Growth factor receptors may be transactivated not only by homologous receptors, but also by heterologous receptors. We have investigated this possibility, using for this purpose R-/EGFR cells, which are mouse embryo cells devoid of IGF-I receptors, but overexpressing the EGF receptor. At variance with mouse embryo cells with a wild-type number of IGF-I receptors and overexpressing the EGF receptor, R-/EGFR cells cannot grow in EGF only, nor can they form colonies in soft agar. However, if a wild type human IGF-I receptor is stably transfected into R-/EGFR cells, growth in EGF and colony formation in soft agar are restored. To determine a possible interaction between the two receptors, we transfected into R-/EGFR cells a number of IGF-I receptor mutants with different impaired functions. The only IGF-I receptor that cannot reverse the growth phenotype of R-/EGFR cells is a receptor with a point mutation at the ATP-binding site. All other mutant receptors, even when incapable of responding to IGF-I with a mitogenic signal, made R-/EGFR cells fully capable of responding with growth to EGF stimulation. IGF-I receptor mutants that are mitogenic but not transforming made R-/EGFR cells grow in EGF only, but were incapable of inducing the transformed phenotype. The mutant IGF-I receptors are activated (tyrosyl phosphorylation of IRS-I) in response to EGF. These experiments indicate that certain IGF-I receptor mutants with loss of function can be reactivated intracellularly by an overexpressed EGF receptor and confirm that the C-terminus of the IGF-IR is required for its transforming activity.