Mouse embryo cells expressing a wild-type number of insulin-like growth factor I receptors (IGF-IR) (W cells) can be transformed either by simian virus 40 large T antigen (SV40 T) or by overexpressed insulin receptor substrate 1 (IRS-1), singly transfected. Neither SV40 T antigen nor IRS-1, individually, can transform mouse embryo cells with a targeted disruption of the IGF-IR genes (R ؊ cells). However, cotransfection of SV40 T antigen and IRS-1 does transform R؊ cells. In this study, using different antibodies and different cell lines, we found that SV40 T antigen and IRS-1 are coprecipitated from cell lysates in a specific fashion, regardless of whether the lysates are immunoprecipitated with an antibody to SV40 T antigen or an antibody to IRS-1. The same antibody to SV40 T antigen, however, fails to coprecipitate another substrate of IGF-IR, the transforming protein Shc, and two other signal-transducing molecules, Grb2 and Sos. Finally, an SV40 T antigen lacking the amino-terminal 250 amino acids fails to coprecipitate IRS-1 and also fails to transform R ؊ cells overexpressing mouse IRS-1. These experiments indicate that IRS-1 associates with SV40 T antigen and that this association plays a critical role in the combined ability of these proteins to transform R ؊ cells. This finding is discussed in light of the crucial role of the IGF-IR in the establishment and maintenance of the transformed phenotype.The insulin-like growth factor I (IGF-I) receptor (IGF-IR) (56) plays a crucial role in the establishment and maintenance of the transformed phenotype. Antibodies to IGF-IR (1, 18), antisense expression plasmids to either IGF-I (55), IGF-IR (4, 40, 41, 48), or IGF-II (6), and dominant negative mutants of IGF-IR (25, 37) can all reverse the transformed phenotype and/or inhibit tumorigenesis. Conversely, overexpression of the wild-type (but not a mutant) IGF-IR induces transformation (7,19,25,28,46), while overexpression of IGF-II in transgenic mice increases the incidence of certain malignancies (42).Recently, we have generated from mouse embryos homozygous for a targeted disruption of the IGF-IR genes and from their wild-type littermates (2, 26) cell lines of 3T3-like fibroblasts, designated, respectively, R Ϫ (receptor minus) and W (wild-type) cells (46,47). R Ϫ cells grow in 10% serum (albeit more slowly than W cells) but do not grow at all in serum-free medium supplemented with the growth factors that sustain the growth of W cells, and of other 3T3-like cells, such as plateletderived growth factor (PDGF), epidermal growth factor, IGF-I, IGF-II, insulin at supraphysiological concentrations, and fibroblast growth factors (7, 46). In addition, R Ϫ cells are refractory to transformation by simian virus 40 (SV40) T antigen (47) or by other oncoproteins, such as an activated HaRas (46, 52), singly or in combination. The growth deficits of R Ϫ cells, including their resistance to transformation, are abrogated by the stable transfection of a plasmid expressing a wild-type (but not a mutant) human IGF-IR cDNA (7,9,25...