The efficacy and safety of a low molecular weight heparin (Kabi 2165) in preventing postoperative deep vein thrombosis (D.V.T.), was assessed in a double blind randomly allocated multicenter trial. 385 patients were included and analysed on a intention to treat basis. Kabi 2165 was given S.C. 24 hourly in 2 500 anti-factor Xa units and compared with standard low dose calcium heparin 5 000 i.u. S.C. 12 hourly in patients undergoing major abdominal or gynaecological surgery. The first dose was administered two hours before operation in both groups. The relevant characteristics of the patients in the two treatment groups were similar. The two groups were well matched for risk factors which could predispose to D.V.T.DVT was detected by the radioactive fibrinogen test. Venography was performed whenever a positive scan developed in a patient. Six (3,1 96) of 195 patients receiving Kabi 2165 and seven (3,7 96) of 190 patients in the standard heparin group developed D.V.T. No pulmonary embolism we re detected during the prophylactic regimens. There was no significant difference between the two groups in terms of blood loss during surgery, postoperative drainage, blood transfusion, wound haematoma. Mean hemoglobin levels and mean hematocrit values preoperatively and postoperatively (day 1 and 6) were :There were no statistically significant differences in both groups. No thrombocytopenia was reported in this study. The antifactor Xa activity was significantly higher in the Kabi 2165 group.In conclusion, Kabi 2165 once daily is as effective and safe as standard heparin twice daily in preventing postoperative D.V.T. in general surgery.