Summary The modulation of proliferation and differentiation in primary epidermal keratinocyte cultures by lowered gas phase oxygen tensions was studied. Neonatal mouse epidermal keratinocyte cultures were grown in an Heraues type B 5060 EK/O 2 incubator in oxygen tensions between 5% and 15% (within the physiologic range); the oxygen tension of ambient air being 21%. Cell morphology was studied using histochemical stains and electron microscopy. Differentiation was assessed using autoradiography of SDS PAGE gels of six serially extracted cell protein fractions with [ 3H]leucine as a marker. Autoradiographs using [ 14C]glucosamine and 32P i as markers were also assessed as a measure of other cell functions. Proliferation was studied using autoradiography of [ 3H]thymidine ([ 3H]TdR) pulse-labeled cultures and [ 3H]TdR incorporation into isolated DNA fractions. The results of these studies showed that lowering the oxygen tension in the gas phase reversibly inhibited cell proliferation. There was a direct arithmetic relationship between the proliferative rate of the cultures and the oxygen tension. No change in differentiation as defined by [ 3H]leucine indexing of protein synthesis was seen. Other markers of cell function, such as [ 14C]glucosamine glycosylation and [ 32P] phosphorylation of proteins were also unchanged. These results suggest that oxygen tension regulates only proliferation in epidermal keratinocytes. This epidermal response is well adapted to its role in the healing wound, and is an example of a tissue-specific modification of a regulatory function.