The influence of soybean galactosaccharides (stachyose, raffinose, melibiose) as well as galactose and glucose on the synthesis and activity of alpha-galactosidase (alpha-gal) from Lactobacillus fermentum CRL 251 was studied. Stachyose was the most effective inducer, followed by melibiose, raffinose, and galactose; scarce activity was detected with glucose. Exogenously supplied glucose inhibited the synthesis of the enzyme in cultures of L. fermentum growing on galactose. This effect was reversed by the addition of cyclic adenosine-3',5' monophosphate (cAMP), which suggests that this compound could be involved in the regulation of alpha-gal synthesis.