Human liver cytoplasmic aspartate aminotransferase was found to exhibit five subforms with isoelectric points of 5.15, 5.30, 5.45, 5.60, and 5.80. Treatment with neuraminidase did not affect their electrophoretic mobility. The immunochemical and steady-state kinetic properties of the subforms were identical. Heat treatment increased the proportion of acidic subforms, but all forms were present in fresh tissue. 2-Mercaptoethanol or inhibitors of proteolysis failed to protect against the formation of the subforms with lower isoelectric points. Multiple molecular forms with similar properties were found for the enzyme of human erythrocytes. This evidence is consistent with deamidation of asparaginyl or glutaminyl residues as the origin of the multiple forms. Human mitochondrial aspartate aminotransferase presented as a single molecular form with an isoelectric point of 9.7.