Spontaneously apoptotic U937 cells from exponentially growing cell cultures were enriched on discontinuous Percoll density gradients. Increased PKC-beta and reduced PKC-zeta expression were detected in apoptotic cells by Western blotting. Using confocal microscopy, changes in the level of PKC isoenzymes were confirmed and in addition alterations in the subcellular location of PKC isoenzymes were detected in apoptotic cells compared with nonapoptotic cells. The data indicate that the expression of specific PKC isoenzymes is modulated during apoptosis and that PKC-beta and PKC-zeta may play specific roles in the regulation of the apoptotic program.