Six activated partial thromboplastin reagents from four manufacturers were evaluated for precision, sensitivity, and dose responsiveness to Factors VIII, IX, and XII. Normal and abnormal APTT samples were prepared. Precision was assessed using 10 replicates on three consecutive days. Factor sensitivity and responsiveness were measured using a log-log curve of percent activity versus the upper limit of normal of prepared samples. All reagents had acceptable precision (C.V. less than 5%) in the normal range, but three performed unacceptably in the abnormal range. Two of the reagents were determined to be too sensitive to certain factors, giving an abnormal APTT result when the factor activity was in the normal range. Sensitivity and dose responsiveness to factors should be considered as two separate characteristics of an APTT reagent.