Distillers’ solubles, the liquid residue obtained after distillation of fermented mash, are often recycled as backset to partially replace process water and to reduce effluent treatment costs. In this study, wheat mashes were artificially infected with lactic acid bacteria. Successive recycling of laboratory distillers’ solubles obtained from contaminated mashes was carried out. Parameters were monitored over five successive fermentations in order to study the increasing effects of backset contamination. Contamination of wheat mash with up to 6 x 10(8) lactic acid bacteria per milliliter did not seriously impede ethanol productivity of the yeast. The alcohol yields ranged from 55 to 56 gl(-1) even though the accumulation of lactic acid in distillers’ solubles contaminated with L. delbrueckii reached as high as 14 gl(-1) after five recyclings. Although the gradual buildup of the acid over five successive fermentations did result in the loss of as much as 60% of the yeast viability, death of the yeast late in the last two fermentations did not affect yeast fermentative ability or the activity of glucoamylase.