Two types of LMW heparin were prepared by gel filtration of standard heparin (LMW fraction) and by degradation of heparin by nitrous acid (LMW fragment), respectively. The effects on factor Xa inhibition (XaI), APTT, platelet aggregation and AT III level of these preparations were studied after subcutaneous administration to humans and compared with those of standard heparin. At a dose of 5000 IU (XaI) the LMW fraction and LMW fragment induced peak plasma XaI activity of 0.32 IU/ml and 0.41 IU/ml respectively, compared to 0.07 IU/ml for heparin. Still 11.5 h after administration both LMW preparations gave higher activities than heparin ever induced. Following administration of 10,000 IU (XaI) of the LMW fragment the plasma peak XaI activity was 0.81 IU/ml. This prolonged the APTT from 36 sec to 46 sec only. The half-lives of the XaI activity in plasma were between 3 and 4 hours. No effect on platelet aggregation or AT-III level was demonstrated.