The enzyme ß-galactosidase (EC 3.2.1.23) from Aspergillus niger was purified and resolved into three multiple forms, using molecular sieving, ion-exchange, an hydrophobic chromatography. The isolated enzyme forms accounted for 83%, 8%, and 9% of the total ß-galactosidase activity, respectively. They were glycoproteins with estimated molecular weights of 124,000, 150,000 and 173,000, isoelectric points of about 4.6, and pH optima between 2.5 and 4.0. Amino acid and carbohydrate analyses showed that multiplicity was mainly due to dissimilar carbohydrate contents (about 12.5%, 20.5% and 29% neutral carbohydrates, respectively). The multiple form pattern might depend on the culture conditions. The ß-galactosidase forms were heat-stable up to about 60 degrees C. The Km values for lactose ranged from 85 mM to 125 mM, whereas those for the synthetic substrate o-nitrophenyl-ß-D-galactopyranoside were equal to about 2.4 mM. The V values obtained at 30 degrees C for lactose and o-nitrophenyl-ß-D-galactopyranoside were 104 units/mg enzyme protein and 121 units/mg enzyme protein, respectively (weighted averages for the three enzyme forms). The slight reactional dissimilarities between the three enzyme forms are unlikely to be physiologically relevant. The biological significance of A. niger ß-galactosidase multiplicity might be related to the observed differences in carbohydrate content, as suggested by recent reports on other microbial glycoprotein enzymes.