An international collaborative study has been carried out with the aim of establishing an international standard for low molecular weight (LMW) heparin. Three preparations of LMW heparin were assayed against the International Standard for unfractionated heparin (UFH) by 25 laboratories in 13 countries, using nine different assay methods. The results confirmed previous findings of non-parallel assays, wide interlaboratory variability and differences between methods when LMW heparins are assayed against a UFH standard. Use of one of the LMW heparins as a standard for the other two gave parallel assays and much closer agreement between laboratories. The preparation in ampoules coded 85/600 was selected as likely to give the best agreement with the largest number of LMW heparins; potencies were assigned by taking the mean of all the anti-Xa assays, and the mean of the thrombin and APTT assays, to represent the two major groups of activities. Preparation 85/600 has been established by WHO as the 1st International Standard for LMW heparin, with potencies of 1,680 iu/ampoule by anti-Xa assays and 665 iu/ampoule by thrombin inhibition and APTT assays.