Effects of a bacterial probiotic (BP) on ruminal fermentation and plasma metabolites were evaluated in four Holstein cattle (body weight, 645 ± 62 kg; mean ± SD) with induced subacute ruminal acidosis (SARA). SARA was induced by feeding a SARA-inducing diet, and thereafter, 20, 50 or 100 g per head of a commercial BP was administered for 7 consecutive days during the morning feeding. Cattle without BP served as the control. The 24-hr mean ruminal pH in the control was lower, whereas those in the BP groups administered 20 or 50 g were significantly higher compared to the control from days 2 to 7. Circadian patterns of the 1-hr mean ruminal pH were identical (6.4-6.8) among all cattle receiving BP. Although the mean minimum pH in the control on day -7 and day 0 was <5.8, the pH in the treatment groups on day 7 was >5.8 and significantly higher than that of the control group ( >5.2). Ruminal volatile fatty acid (VFA) concentrations were not affected by BP treatment; however, the BP groups had lower lactic acid levels compared with the control group at 20:00 on day 7. Additionally, non-esterified fatty acid levels decreased from 8:00 to 20:00 in all BP groups on day 7. These results suggest that administration of 20 to 50 g of a multi-strain BP for 7 days might improve the low pH and high lactic acid level of the ruminal fluid in SARA cattle.