FCE 22891, a synthetic beta-lactam antibiotic of the penem class, was administered by gavage to Sprague-Dawley rats and cynomolgus monkeys for 26 wk (with and without a 6-wk recovery). Rats received the test material at doses of 0, 200, 500, and 1,250 mg/kg/day, and monkeys were given doses of 0, 100, 200, 400, and 600 mg/kg/day. At the end of the 26-wk treatment period, approximately two-thirds of the animals (both species) were sacrificed, and the remaining animals were held without treatment for a further 6 wk. A treatment-related mortality occurred in female monkeys receiving 600 mg/kg. There was a reduction in body weight gain in the high-dose groups of both species. Male rats were more affected than the females and, conversely, female monkeys were affected more than the males. At higher dose levels, both species exhibited an early, but transient, azotemia and oliguria with an increase in specific gravity and reduced pH. In rats, microscopic examination revealed treatment-related renal cortical tubular degenerative and regenerative changes with associated interstitial inflammation and fibrosis and diffuse urothelial hyperplasia in the urinary bladder. In general, female rats were less severely affected, and in both sexes there was a trend to recovery of most of these effects. In monkeys given 600 mg/kg of the test material, renal cortical tubular degeneration was seen only in those females that died in the first 5 wk of dosing. In other animals at this dose level, the renal lesions were determined to be reversible.