Low-molecular weight heparin (LMWH) is the product of enzymatic or chemical degradation of unfractionated heparin (UFH). It has been found to have better bio-availibility, more predictable dose response and can be used as an alternative to UFH for prophylaxis and treatment of thrombotic disorders. It is claimed that no laboratory monitoring is necessary for LMWH therapy; however, for the aged, renal impaired, obese or grossly underweight, monitoring of dose effect with anti-Xa assay is recommended. The activated partial thromboplastin time (APTT), which is the test of choice for UFH monitoring, is believed to be insensitive to the effect of LMWH. The sensitivity of the APTT to heparin lies in the APTT reagent used. In this study, eight different APTT reagents were used to compare the APTT with anti-Xa activity in ex vivo plasma from patients who were on enoxaparin (LMWH, Clexane) therapy. It was found that, as with UFH, APTT reagents show variable sensitivity to LMWH. The APTTs from all eight reagents were found to have a linear relationship to anti-Xa activity. The APTT results using three of the reagents gave an indication of the use of LMWH therapy. It was also found that patients who were lupus anticoagulant (LA)-positive had much more prolonged APTTs when on LMWH therapy; however, a linear correlation between APTT and anti-Xa was not present in these patients.