CCAAT/enhancer binding proteins (C/EBPs) are a highly conserved family of DNA-binding proteins that regulate cell growth and differentiation in a highly tissue-specific manner. These experiments investigated the influence of the cell cycle on C/EBP isoform expression in mammary epithelial cells (COMMA D) and fibroblasts (NIH3T3). C/EBP-delta gene expression is induced in COMMA D cells arrested in G0 by serum and growth factor withdrawal or contact inhibition. C/EBP-delta mRNA, nuclear protein content, and DNA binding activity increase during G0 growth arrest and decrease after cell cycle induction in COMMA D cells. Growth arrest is markedly delayed in COMMA D cells expressing a C/EBP-delta antisense construct. C/EBP-beta is induced during G1 of the cell cycle. In contrast to COMMA D cells, C/EBP-beta and C/EBP-delta mRNA levels remain relatively constant in growth-arrested and cell cycle-induced NIH3T3 cells. However, C/EBP homologous protein (CHOP10) mRNA levels markedly increase in growth-arrested NIH3T3 cells. Both COMMA D and NIH3T3 cells express growth arrest-specific (gas1) and JunD during G0. These results demonstrate that COMMA D and NIH3T3 cells achieve a common growth arrest (G0) state by cell-specific strategies that involve the induction of different C/EBP isoforms.