Protein carboxyl methyltransferase (EC 2.1.1.77) which has specificities for the protein contains abnormal aspartyl residues is investigated in lysosome, mitochondria, peroxisome of age-dependent rat liver, alloxan-diabetic rat liver and vacuole in beet root. The level of this enzyme in lysosome, mitochondria, and peroxisome of rat liver increased according to "ageing". Methyl accepting abilities were also increased. In allosan-diabetic rats and vacuole vesicles the enzyme activities and methylatabilities increased. The physiological significance of this methylation is not clear, but PCM has been proposed to play a role in the repair or degradation of damaged proteins. By CPC-PAGE, 38 kDa in lysosome and 19, 30, 38 kDa in mitochondria and peroxisome were found to be a good substrate for PCM.