Bifidobacterium infantis HL96 produces three beta-galactosidases (beta-gal I, II and III). A genomic bank of B. infantis was constructed in E. coli by using pBR322 as a cloning vector. Two E. coli transformants, BIG1 and BIG4, possessing beta-galactosidase activity, were selected from X-gal plates. They contained two different recombinant plasmids with insert DNA fragments of approx. 4.6 and 4.4 kb, respectively. The restriction maps of pBIG1 and pBIG4 were constructed. beta-Galactosidases from crude cell-free extracts of B. infantis and of two E. coli recombinants were analyzed by native PAGE and characterized by activity staining, pBIG1 and pBIG4 were shown to carry the genes for beta-gal I and beta-gal III, respectively. Optimal pH and temperature for hydrolytic activity of the native enzyme were 7.5 and 40 degrees C, while those for recombinant BIG1 and BIG4 were 7.5, 50 degrees C and 8.0, 40 degrees C, respectively.