The variations between different lots of activated partial thromboplastin time (APTT) produced by three major North American suppliers were evaluated over the past eight years. The authors found significant variations between the heparin sensitivity of the APTT reagents produced under the same name by the same supplier. The variations were so much that, using the recommended APTT ratio or prolongation of APTT for monitoring heparin therapy, one would have achieved significantly different intensity of heparinization from year to year.