The activated partial thromboplastin time (aPTT) is the most popular test for monitoring of heparin therapy. The purpose of the present study was to show that an aPTT reagent with good response to heparin can be prepared from synthetic phosphoglycerides. Mixed liposomes were prepared from synthetic dioleoylphosphatidylserine (DOPS), dioleoylphosphatidylcholine (DOPC), and dioleoylphosphatidylethanolamine (DOPE). These liposomes were used in an aPTT test system with kaolin as activator, to evaluate their procoagulant activity in the absence and presence of heparin. For comparison, mixtures of purified non-synthetic phospholipids were prepared and tested with the same systems. The aPTT and its response to heparin were influenced by the phospholipid class composition and concentration. The presence of phosphatidylserine (PS) was required to reduce the aPTT of normal plasma to values between 30 and 40 s. The presence of phosphatidylethanolamine (PE) in mixed liposomes could modulate the response to heparin. At low PE/PS liposome concentrations (˜ 40 μM), a relatively low response was observed. At high liposome concentrations (˜ 1 mM), the response to heparin increased with the mole fraction of phosphatidylethanolamine. The results obtained with non-synthetic phospholipid mixtures were similar to those obtained with the synthetic phosphoglycerides. Optimal concentrations of DOPS, DOPE and DOPC were found with which an almost linear response to heparin and to low molecular weight heparin (FragminTM) was observed. Using a mixed liposome consisting of 12 μM DOPS/12 μM DOPC/16 μM DOPE, a doubling of the base-line aPTT was achieved at approximately 0.2 IU/ml of heparin, and at approximately 1.0 IU/ml of Fragmin. As the chemical composition of the synthetic phospholipid reagent is well defined, it may constitute a step forward in the standardization of monitoring heparin therapy.