Ubidecarenone, also known as CoQ(10), is currently sold as a dietary supplement in the United States, with a majority of these products derived from the fermentation of carbohydrates or tobacco leaf extracts. In addition to its availability in dietary supplements, CoQ(10) is now being considered for use in foods. Accordingly, as part of the process for attaining "Generally Recognized as Safe" status, and to supplement information already available regarding the safety of CoQ(10) per se, a 28-day oral toxicity study in rats was conducted to evaluate the subacute safety of a microorganism biomass used as a new source in CoQ(10) production. Groups of Crj:CD(SD) rats (SPF) (6 males or females per group, 4 groups per sex) received dried microorganism at doses of 0, 500, 1000 or 2000 mg/kg/day via intragastric intubation. Clinical observations were recorded, and body weight, and food and water consumptions measured throughout the study. At the end of the study, aortic blood samples were collected from all animals for analysis of hematological and clinical chemistry parameters, and gross pathologic examination was performed. Histopathologic examination was performed on select tissues from the control and high-dose groups. There were no treatment-related changes that were considered to be of toxicological significance. Since rats treated with 2000 mg/kg of dried microorganism did not demonstrate any treatment-related changes, the no-observable-adverse-effect level (NOAEL) for dried microorganism was estimated to be greater than 2000 mg/kg/day under the present study conditions.