A caesarean section was performed on 30 cows before normal term and 16 to 20 hours after the induction of parturition with dexamethasone. During the surgical procedure, 20,000 U of bacterial collagenase was injected into the uterine artery of 15 of the cows. The average periods of retention of the fetal membranes were 40 hours in the treated cows and 114 hours in the control cows (P<0.001). At 36 hours after the surgery six of the treated cows (40 per cent) but all 15 of the control cows had retained fetal membranes. The collagenase-treated cows showed no abnormal clinical signs during the 10 days after the operation.