The authors evaluate the follow-up of the anti-Xa level during the prophylactic administration of two different low molecular heparins (enoxaparin, reviparin) in 40 patients with a planned resection of the large intestine, resection of the pancreas, laparoscopic cholecystectomy and laparotomic cholecystectomy. The assessed amounts of anti-Xa did not reach recommended levels even during the perioperative period. The selection of the preparation and the extent of the operation did not influence the anti-Xa levels.