Two convenient high-performance liquid chromatography procedures are proposed for the determination of phenprocoumon (Marcumar) in serum (S-PPC) and its unbound fraction in serum water (SW-PPC). S-PPC and SW-PPC were determined in samples from 53 patients undergoing oral anticoagulant therapy who were within the therapeutic interval of the thrombotest. S-PPC concentrations ranged from 0.6 to 5.0 mg/l, SW-PPC from 1.4 to 21.1 micrograms/l. A linear correlation (r = 0.714) was found between S-PPC and SW-PPC concentrations in the patient samples, the mean free fraction being 0.30%. S-PPC and SW-PPC concentrations were intra-individually stable during therapy. A linear correlation (r = 0.710) was observed between S-PPC concentrations and dosage.