A culture of Lactobacillus delbrueckii ssp. bulgaricus 11842 was investigated as a supplement to stimulate growth in milk of a probiotic culture, Lactobacillus rhamnosus RO11 The L. bulgaricus 11842 culture was subjected to homogenization in order to obtain a crude cellular extract (CCE) containing beta-galactosidase and protease activities. The highest level of beta-galactosidase in the CCE was reached after two passes while the protease activity increased with the number of passes. Fermentations under pH control were carried out with L. rhamnosus RO11 in milk, supplemented with the freeze-dried CCE added to milk at 2gl(-1). Hydrolysis of lactose by the beta-galactosidase in the CCE was evidenced by the appearance of free galactose in the milk. The fermentation rate was increased significantly, almost matching that achieved with glucose and peptone supplementation. Without any supplementation, acid production of L. rhamnosus RO11 in milk was slow. The addition of the CCE to milk also promoted faster acidification in fermentations carried out without pH control, as did the addition of glucose (but not peptone). In these assays, CCE also increased viable counts in the fermented media. The beta-galactosidase activity of the CCE, rather than its proteolytic activity, appeared to be the principal element in the improved acidification rates. The addition of heated CCE to milk failed to significantly improve the fermentation rate, thus confirming the enzymatic activity of the CCE to be the causative agent of its effectiveness. (C) 2004 Elsevier Inc. All rights reserved.