The sensitivity and responsiveness of seven activated partial thromboplastin time reagents to the presence of lupus anticoagulants (LAs) was evaluated with a panel of 50 well-characterized LA plasmas. The results document that some reagents are clearly less responsive and sensitive to LAs; however, there is considerable variability between individual LA samples in these features. In addition, with 16% of these samples, immediate mixing studies showed correction of the activated partial thromboplastin time to the normal range with a 1:1 mixture of normal and LA plasma and 8% to 10% showed either a normal platelet neutralization procedure or a normal tissue thromboplastin inhibition procedure. Together, these findings provide further evidence of the laboratory heterogeneity of these inhibitors. The effect of this variability on the diagnosis of these inhibitors is discussed.