Prospective study implicates problems of heparin-induced thrombocytopenia (HIT). This is one of the most important complications in heparin treatment and prophylaxis. The set was formed of patients with severe diagnosis and larger intra-abdominal operation that was associated with relative high risk of thrombotic event. That's why there was need of heparin prophylaxis. We used Enoxaparin in dose 0.2-0.4 ml subcutaneously. The controlled parameters were platelet counts, detection of antibodies using ELISA Assay method (GTI-PFA), indirect platelet immunofluorescent test for monitoring platelet disorders. We had no known case of clinical symptoms of HIT. The change in platelet counts was due to postoperative course, or binding on blood transfer or basic haematological disease. The test detecting antibodies was positive in eight percent without clinical correlation, without important thrombocytopenia. Low-molecular-weight heparin has certain advantages over unfractionated heparin, as well know from clinical and laboratory monitoring in other studies, including larger safety (in relationship with HIT) with very effective prophylaxis.