Commercial partial thromboplastin reagents markedly differ in their sensitivity to factor deficiency, heparin, or the lupus anticoagulant. These differences may be partly due to the variable phospholipid content of different commercially available reagents. For over 15 years, we have routinely used a partial thromboplastin prepared from human brain. In the past four years, we have been using a similarly prepared bovine partial thromboplastin reagent. This report describes the preparation of our partial thromboplastin reagent, as well as an analysis of the phospholipid composition of both the human and bovine thromboplastin reagents. Four separate brain preparations produced consistent percentages of the anionic phospholipids, phosphatidylserine, and phosphatidylinositol. The bovine reagent was also compared with commercial partial thromboplastin reagents in the detection of coagulation factor deficiency, heparin, and the lupus anticoagulant.