We have investigated the activation of ERK2, a serine/threonine kinase necessary for transmission of mitogenic signals, in cells derived from mouse embryos homozygous for a null mutation of the insulin-like growth factor (IGF)-1R gene (R- cells) and from wild-type littermates (W cells), respectively. Stimulation of quiescent W cells with IGF-1, epidermal growth factor (EGF), or with a combination growth factors induced both a maximal transient and a prolonged activation of ERK2, whereas platelet-derived growth factor or a combination of platelet-derived growth factor and EGF resulted only in transient activation of ERK2. In contrast, stimulation of R cells with IGF-1, EGF, or combinations of growth factors resulted in a transient and submaximal activation of ERK2. Reintroduction of a wild-type human IGF-1R or of a C-terminus IGF-1R mutant, but not of a juxtamembrane mutant IGF-1R, into R- cells was able to restore ERK2 activation to wild-type levels. Thus, prolonged ERK2 activation in mouse embryo fibroblasts stimulated with purified growth factors is largely dependent on a signal generated by the IGF-1R.