The protein compositions of subcellular fractions prepared from untreated and 12-O-tetradecanoyl phorbol-13-acetate (TPA)-treated Krebs II ascites cells were examined by SDS-polyacrylamide gel electrophoresis. After 6 h of TPA treatment a 65,000 dalton protein in the nuclear associated endoplasmic reticulum showed a 240% increase when compared with the untreated control cells. The amount of this protein was similar in both untreated and TPA-treated cells up to 4 h of in vitro incubation. Though present in all other subcellular fractions, the amount of the 65,000 dalton protein was the same in both untreated and TPA-treated cells. Small changes were also observed in the amounts of other proteins in the nuclear associated endoplasmic reticulum in TPA-treated cells. The possibility was tested that the increased amount of the protein may have represented increased ornithine decarboxylase activity in response to TPA treatment. The time dependent appearance of the enzyme in untreated and TPA-treated cells, however, and the behaviour and subcellular localization of the 65,000 dalton protein ruled out the possibility that this protein and ornithine decarboxylase are one and the same molecule.