Stimulation of mesangial cells with insulin-like growth factor-1 (IGF-1) resulted in the rapid tyrosyl phosphorylation of nuclear proteins as indicated by fluorescence microscopy of cells stained with anti-phosphotyrosine antibodies. Immunoprecipitation of nuclear extracts with anti-phosphotyrosine antibodies revealed that IGF-1 induced a transient increase in immunoreactive phosphotyrosine in nuclear proteins of 43, 95, and 160 kDa. Using a double immunoprecipitation protocol, the transcription factor c-Jun was also found to increase in immunoreactive phosphotyrosine in response to IGF-1. A similar pattern of tyrosyl phosphorylation of nuclear proteins was observed in the epidermoid carcinoma cell line CaSki. These data suggest that tyrosyl phosphorylation of nuclear proteins may be a step in the transduction of mitogenic signals.