A human ornithine decarboxylase (ODC) gene probe has been isolated from a Jurkat T‐cell cDNA expression library, sequenced, and used to analyze ODC mRNA levels in untransformed human lymphocytes and fibroblasts stimulated to proliferate by various mitogens. The partial cDNA sequence is 86% homologous to the mouse ODC cDNA, and Northern blots indicate that the human and mouse mRNA species are similar in size. ODC mRNA is barely detectable in quiescent human T lymphocytes and undetectable in density‐arrested WI38 fibroblasts. Following stimulation of T‐lymphocyte proliferation with phytohemagglutinin, the ODC mRNA level rises to a peak around mid G₁ phase and decreases as the cells enter S phase. Serum stimulation of density‐arrested fibroblasts results in an elevation of the ODC mRNA level which persists throughout the cell cycle. Epidermal growth factor (20 ng/ml) but not insulin (10 mg/ml) or dexamethasone (55 ng/ml) stimulates ODC expression in quiescent WI38 fibroblasts. Southern blots suggest that human cells have a single copy of the ODC gene.