The human insulinlike growth factor I receptor was overexpressed in NIH 3T3 cells as well as human and rat primary fibroblast strains. The NIH 3T3 cells displayed a ligand-dependent, highly transformed phenotype. When exposed to insulinlike growth factor I or supraphysiologic levels of insulin, NIH 3T3 cells that expressed high levels of receptors formed aggregates in tissue culture dishes, colonies in soft agar, and tumors in nude mice. Expression of 1 million receptors per cell, a 40-fold increase above the base-line level, was required for anchorage-independent growth. Primary fibroblasts that expressed high levels of receptors displayed a ligand-dependent change in morphology and an increase in saturation density but did not acquire a fully transformed phenotype. The results demonstrate that when amplified, this ubiquitous growth factor receptor behaves like an oncogenic protein and is capable of promoting neoplastic growth in vivo.