Various nitrogen sources were compared with yeast extract (YE) in terms of their usefulness for efficient lactic acid production by Lactobacillus casei subsp. rhamnosus. None of these nitrogen sources gave lactic acid concentrations as high as that for YE during 48 h of fermentation. On the basis of the amount of B vitamins in YE, various amounts of five essential B vitamins known to support the growth of L. casei were supplemented together with the nitrogen sources. The addition of B vitamins at less than 25 mg/l enhanced the lactic acid production to some extent in all cases except for the media supplemented with nitrogen sources of casein origin. The most significant increase was obtained with the N-Z-Soy peptone medium, in which the production of lactic acid after 48 h of fermentation was 2.2-fold higher than that without vitamin supplementation. Half of the YE in a medium supplemented with 10 g/l YE could be replaced by B vitamins at less than 25 mg/l and 3.3 g/l N-Z-Soy peptone without a significant decrease in the rate of lactic acid production.