Ex vivo expansion of T cells is an important aspect of many cellular immunotherapy protocols, and the effects of the culture environment on the cells must be understood to produce large numbers of functional cells. Extracellular pH is a fundamental parameter that has many different effects on cultured cells. In this study, peripheral blood mononuclear cells were stimulated with phytohemagglutinin and cultured at pH values of 7.0, 7.2, or 7.4. The effects of pH on the cells were studied during the 2 to 3 weeks of proliferation resulting from phytohemagglutinin stimulation, in order to examine the culture kinetics over realistic time scales for ex vivo expansion. The proliferation capacity of the T cells increased more than three-fold for the pH 7.0 and 7.2 cultures compared with the pH 7.4 cultures. The culture pH also affected the kinetics of the interleukin-2 receptor down-regulation process. The faster receptor down-regulation in both the pH 7.2 and 7.4 cultures resulted in a more than twofold greater fraction of interleukin-2 receptor+ cells in the pH 7.0 cultures. Although the fraction of apoptotic cells (using the Annexin V flow-cytometric method) remained less than 10%, we observed 27% more apoptosis in the pH 7.4 cultures than in the 7.2 cultures and 49% more apoptosis in the pH 7.4 cultures than in the 7.0 cultures. These effects on interleukin-2 receptor expression and cellular apoptosis may partially explain the observed effects of pH on T-cell proliferation.