Prevention of intraperitoneal fibrin formation has attracted attention in attempting to limit development of peritoneal adhesions. Operations on the pelvis for infertility were performed upon 92 patients, in all of whom the exposed peritoneal serosa was irrigated throughout with warm isotonic Ringer's lactate solution. In 44 randomly assigned patients, heparin 5,000 International units per liter was added to the irrigating solution. Forty-eight patients received no heparin. Laparoscopy was carried out in all patients within 12 days of operation in order to diagnose and treat pelvic adhesions. Analysis of adhesion improvement scores in relation to initial adhesion scores showed that no significant benefit was obtained from the use of heparin. The probability that an important beneficial effect of heparin was missed in the study was slight (p less than 0.03, 1-tailed). It is concluded that the use of heparin to irrigate the peritoneal serosa during elective operations upon the pelvis has no important action in reducing the development of peritoneal adhesions.