Id1, Id2, and Id3 (HLH462) dimerize with members of the basic helix-loop-helix protein family, but due to the absence of the basic region, the resulting heterodimers cannot bind DNA. Therefore Id-type proteins negatively regulate DNA binding of the basic helix-loop-helix proteins. Here we report that Id1, Id2, and Id3 are induced shortly after serum stimulation in arrested NIH 3T3. Antisense oligonucleotides against the Id mRNAs delay the reentry of arrested cells into the cell cycle elicited by stimulation with serum or growth factors. Antisense oligonucleotides against all three Id mRNAs are more effective than individual ones. Combined, these results indicate that Id proteins are involved in the control of growth induction.