The antibiotic Virginiamycin (Vm) can be used at mu g levels to suppress gram-positive bacteria during alcoholic fermentation. The stability of Vm was studied during 96 h ethanol production by Saccharomyces cervisiae for two different media in a 24 liter fermentor at 30 degrees C. The Vm levels were: 0, 2, and 20 ppm, in malt glucose yeast extract broth (MGYEB) and hydrolyzed cornflour (HCF). The antibiotic was assayed in media before and during fermentation, in distilled ethanol and in the stillage (slop). Vm was assayed by Micrococcus luteus ATCC 9341 inhibition of citric acid-acetone extracts of all samples. Glucose consumption rates (g liter(-1) h(-1)), at 12 h fermentation, were not significantly different in the control and in the presence of 2 ppm Vm for both media (3.43 g liter(-1) h(-1) in MGYEB and 3.79 in HCF) but 20 ppm Vm, suppressed the rates (2.28 g liter(-1) h(-1) in MGYEB and 2.52 in HCF). Average Vm recovery from samples was 86.6% with a coefficient variation (CV) of 5.8% before fermentation in both media and 58.4% with CV of 3.9% in wort after complete fermentation. Distillation of inactivated Vm and stillage contained only 13.2% of original Vm level. When HCF stillage was heated (30 min at 100 degrees C), Vm activity was reduced to 2.6% of original. During the assay of Vm in recovered ethanol, suppression of Micrococcus luteus growth was observed, equivalent to growth suppression by 15-45 ppb Vm but this was not because of Vm. Two ppm Vm exhibited no effect on ethanol production and can be eliminated from the by-product slops by adequate heating. Copyright (C) 1996 Elsevier Science Ltd.