A comparative analysis of protein kinase C-theta (PKCtheta) protein expression was performed in various mouse organs and tissues, freshly isolated populations of mouse and human hematopoietic cells, primary leukemias, and established cell lines of different histological origins. Results demonstrated a predominant expression of PKCtheta in lymphoid tissues and skeletal muscle. Expression levels of PKCtheta, as well as PKCalpha, delta, epsilon, zeta, and eta in the thymus, were not markedly changed during postnatal development. High levels of expression were observed in CD4(+) and CD8(+) single-positive T cells and CD4(+)CD8(+) double-positive thymocytes, while B cells were completely devoid of PKCtheta. PKCtheta was found also in platelets, but relatively low levels or no detection of PKCtheta expression were observed in neutrophils, monocytes, and macrophages. Highly proliferating leukemic T cells of established lines or primary tumors, but not freshly isolated resting peripheral blood T cells, exhibited high levels of membrane-bound PKCtheta. Increased proportions of PKCtheta in the particulate fraction was not restricted to malignant cells but correlated with the extent of proliferation of the T cells. Thus, human peripheral blood T cells that were induced to proliferate by exposure to mitogen and IL-2 expressed increased levels of PKCtheta in the particulate fraction. Significantly lower proportions of membrane-bound PKC were observed for five other isoenzymes expressed in T cells. The occurrence of PKCtheta in T, but not B, cells and its subcellular distribution in proliferating cells implicate PKCtheta in cellular mechanisms regulating the sustained proliferation of T cells.