Hexosaminidase (Hex) BA was prepared from purified placental Hex A by treatment with merthiolate and rechromatography on DEAE Sepharose 6B-CL. The heat stability, isoelectric focussing pattern, and peptide map of this isoenzyme were compared with those of naturally occurring purified placental Hex B. Hex BA proved to be less heat stable at 60 °C, have a slightly lower pI, and have one more peptide spot than did Hex B. The lower pI of Hex BA is not caused by sialic acid residues since it was unaffected by neuraminidase treatment. The extra peptide spot in the Hex BA map cannot be explained by carbohydrate differences since no corresponding unpaired peptide spot was seen in the Hex B map. Though Hex B and BA are products of the same gene, there is evidence that both are derived from a larger precursor polypeptide chain. Our results indicate that Hex B and BA are processed differently resulting in a small peptide present in Hex BA that is not found in Hex B