beta-Galactosidase from Saccharomyces lactis has been purified to serve as a model for the kinetic behavior of human lactase in adult lactase deficiency. Enzymes from both species are neutral and follow Michaelis-Menten kinetics. beta-Galactosidase of S. lactis is more readily available than the human lactase. An enzyme preparation from S. lactis (Maxilact 40,000), which is used commercially to hydrolyze lactose in milk, has been found to contain four isozymes of beta-galactosidase. Methods have been developed for the separation and purification of each of the four enzymes. The enzymes were found to differ in molecular mass, kinetic behavior, isoelectric point, response to pH, specific volume and sensitivity to metal ions. The four enzymes had apparent molecular masses of 630 kDa, 550 kDa, 41 kDa and 19 kDa. Their specificity constants (kcat/Km) were found to be 42.0, 355.2, 0.38 and 0.48 mM-1 s-1, respectively. The techniques of reiterated ultrafiltration used for the isolation of these isozymes may be applicable to other purification processes.