Studies were conducted to examine the stabilities of virginiamycin (Vm) and penicillin-G (PG) during incubation at 25 or 35 degrees C in sterile malt-glucose-yeast-extract (MGYE) fermentation broth and during alcohol fermentation by Saccharomyces cerevisiae, Davis 522 strain. A 5 x 2 (five pH levels and two temperatures) factorial design was used to evaluate the stabilities of these two antibiotics. The pH of MGYE used were: 3.8, 4.0, 4.2, 4.5, and 4.8, to which standard concentration of Vm and PG was each added to yield 0.6 mu g Vm ml(-1) and/or 0.5 unit PG ml(-1). Each antibiotic was assayed during incubation at intervals (0-72 h) using Micrococcus luteus. Only Vm (0.6 mu g ml(-1) at pH 4.8) was used during the alcohol fermentation at 35 degrees C, The concentration of Vm in the fermentation broth was not significantly altered at both incubation temperatures as a function of time and at all pH values, whereas PG was rapidly inactivated. The inactivation of PG was significantly pronounced at 35 degrees C compared to 25 degrees C and in direct proportions to the pH. At pH 4.8, the biological half-life of PG in the fermentation broth was 24 and 4 h at 25 and 35 degrees C, respectively, whereas at pH 3.8, it was <4 h for both temperatures. During the alcohol fermentation (72 h) at 35 degrees C, the concentration of Vm (0.6 mu g ml(-1), pH 4.8) was not significantly altered.