The isoelectric points of mitochondrial and cytosolic alanine aminotransferases (EC 2.6.1.2) of bovine brain cortex, of rat, guinea-pig and human livers and of rat and pig kidney cortices were estimated. 2. The pI of the cytosolic enzymes were found at about 5.2 and those of mitochondrial ones at 7.2. 3. It is suggested that in the examined tissues except for the human liver there are two different proteins with the alanine aminotransferase activity.