beta-Galactosidases from Lactobacillus delbruekii subsp. bulgaricus 20056, Lb, casei 20094, Lactococcus lactis subsp. lactis 7962, Streptococcus thermophilus TS2, Pediococcus pentosaceus PE39 and Bifidobacterium bifidum 1901 were partially purified. The rate of hydrolysis of lactose given by the predominant beta-galactosidase activity from each of the bacteria studied was in all cases enhanced by Mg2+, while the effect of K+ and Na+ differed from strain to strain. The beta-galactosidases from all strains also catalysed transgalactosylation reactions. The types of oligosaccharides produced appeared to be very similar in each case, but the rates of their production differed. All the beta-galactosidases were also capable of hydrolysing galactosyl-lactose although, unlike the other bacteria studied, Lb. delbruekii subsp. bulgaricus icus 20056 and Lc. lactis subsp. lactis 7962 were unable to utilise galactosyl-lactose as a carbon source for growth.