AbstractWe have examined the expression and responses to activation, of novel/atypical protein kinase C (PKC) isoforms ε,ζ, and δ in the T cell lymphoma cell line K‐4. The effects of 1‐h phorbol 12‐myristate 13‐acetate (PMA) and OKT3 activation of K‐4 cells on PKC isoform distribution were examined. In addition, the effects of PMA‐mediated down‐regulation on the expression of PKC ε and ζ were determined using high concentrations of PMA over 24‐ and 48‐h time periods in these cells. PKC ε expression was not altered by incubation of K‐4 cells with up to 200 ng/ml PMA over a 24‐ or 48‐h period. PKC ε was down‐regulated in a concentration‐dependent manner by PMA after both 24‐ and 48‐h of activation. Expression of PKC ε was not completely depressed, however, even at the highest concentration of the phorbol ester after 48‐h incubation with PMA. The presence of PKC ε, ζ, and δ was confirmed by immunohistochemistry with distinct patterns of expression observed. PMA‐induced PKC activation for a 1‐h period resulted in a translocation of PKC δ from resting cytoplasmic/nuclear staining to a cytoplasmic aggregate. Following 1‐h activation through the T cell receptor‐associated complex CD3, PKC δ translocated from a peri‐nuclear/cytoplasmic compartment to a putative cytoskeletal location in K‐4 cells. This translocation was time dependent and redistributed to a cytoplasmic aggregate prior to the cytoskeleton. Similarily, following 1‐h activation through the T cell receptor, PKC ζ redistributed directly to what is possibly a cytoskeletal cell compartment. The cytoplasmic distribution of PKC ζ was unaltered following activation with PMA over a 1‐h time period. There was no apparent redistribution of PKC ε cytoplasmic staining pattern following a 1‐h direct or indirect activation. These results underline the differences in individual PKC isoform distribution, and responses to different stimuli, thereby providing additional evidence for the use of discrete PKC isoform signaling pathways in T cells. Furthermore, this data underlines the differences in PMA