LMP1 induces the expression of two members of the family of Id proteins, Id1 and Id3, and affects cell cycle regulation by decreasing the expression of the cyclin dependent kinase inhibitor, p27, and increasing levels and phosphorylation of cdk2 and Rb. In the present study, the contribution of the Id proteins to LMP1-mediated transformation was determined. Although LMP1 effectively inhibited p27 expression, the Id proteins alone did not affect expression of p27, cdk2, and Rb. Neither Id1 nor Id3 was sufficient to transform Rat-1 cells and inhibition of Id1 expression did not affect LMP1-induced morphologic transformation of Rat-1 cells or reduction of p27. However, reduced Id expression resulted in smaller foci and impaired the growth rate of Rat-1 cells. These data indicate that overexpression of the Id proteins is not sufficient for the effects of LMP1 on the cell cycle but that inhibition of Id expression does affect the growth of LMP1-transformed and parental Rat1 cells.