Three LMW heparins (LMWH), one unfractionated heparin (UH), and international standards of LMWH and UH were compared in three chromogenic substrate (CS) assays and the ‘Heptest’ clotting assay. With a two-stage CS assay, linear standard curves were obtained in the 0.1–1.0 U/ml range, nearly coinciding for all preparations. With the one-stage CS assays, standard curves were curvilinear and similar for UH and the LMWH groups. In the Heptest assay, standard curves were linear for UH but not for LMWH. Mean recovery of LMWH, added to patients’ plasma samples was 70–98% for the four assays. Variation between individual recoveries was much greater with Heptest (coefficient of variation (CV) 35–44%) than with one-stage CS assays (CV 14–21 %) or two-stage CS assays (CV 7–8 %). For monitoring LMW heparin therapy, CS assays seem preferable to Heptest. The two-stage CS assay had superior accuracy, but the one-stage CS assays were ea