In order to determine whether the tyrosine phosphatase PTP2C dephosphorylates insulin-elicited phosphotyrosine proteins in vivo, we have compared the patterns of protein tyrosine phosphorylation and its reversal in the kidney 293 cell line with those in 293 cell lines overexpressing PTP2C and a catalytically inactive point mutant of PTP2C. In all three cell types insulin caused the rapid tyrosine phosphorylation of a 160 kD protein, which was shown not to be the insulin receptor substrate 1 (IRS-1) and may be the recently described IRS-2, as well as that of a 100 kD polypeptide, which is probably a mixture of the beta subunits of the insulin and insulin-like growth factor I receptors. There was no difference among the three cell lines in the extent of tyrosine phosphorylation or in the rate of its reversal upon insulin withdrawal. These results indicate that PTP2C does not function to dephosphorylate these proteins significantly in vivo.