Malt combing nuts (MCN) is a low value byproduct from the malting industry, it provides a cheap source of nitrogen and vitamins and has potential for being applied as a nutrient supplement in fermentations to produce lactic acid. This work investigates the supplementation of whey permeate with MCN to produce lactic acid by fermentation with Lactobacillus casei, and compares it with fermentations supplemented with yeast extract (YE). The results showed that MCN can be applied successfully as a nutrient supplement to produce lactic acid by fermentation, achieving complete sugar conversion and lactic acid yield similar to YE supplementation. 5% w/v MCN addition was required to achieve a fermentation time of around 55 h in whey permeate containing lactose at a concentration of 55 g/l. This was similar to fermentations with 0.3% w/v YE supplementation. The major advantage of using MCN is that its raw material cost for supplementing a fermentation is many times lower than YE for a comparable fermentation. However, on the other hand, the results showed that the levels of impurities remaining after fermentation are a lot higher. MCN adds much more ash to the fermentation and there is a lot more unused nitrogen remaining at the end of the fermentation. This is undesirable for the production of high purity lactic acid as it leads to increased separation costs. (C) 2002 Elsevier Science Ltd. All rights reserved.