Abstract The level of ornithine decarboxylase ( L-ornithine carboxy-lyase, EC 188.8.131.52) activity has been compared in untransformed and transformed mouse fibroblasts cultured in vitro under various conditions. It was found that a rapid and marked increase in ornithine decarboxylase activity is induced by the addition of serum to cultures limited in their proliferation by prior incubation in media containing low concentrations of serum. This increase in enzyme activity was prevented by inhibitors of RNA and protein synthesis. The enzyme was found to exhibit a high turnover rate that was not substantially altered under varying growth conditions. It was also shown that the steady-state and induced levels of ornithine decarboxylase varied with the extracellular concentration of serum in a characteristic manner for untrasformed as compared with transformed cells. A close correlation between the level of this enzyme and cell proliferation was observed in each cell type.