The tissue thromboplastin activity of porcine lung was separated from an aqueous tissue extract by differential centrifugation. Repeated suspension and sedimentation yielded a thromboplastin preparation free of soluble protein. The most effective delipidation of the lyophilized preparation with minimum denaturation of the protein moiety of thromboplastin was achieved using n-heptane/n-butanol extraction. A detergent was required to solubilize the thromboplastin protein moiety from the delipidated preparation. Gel filtration in the presence of detergent permitted further delipidation of the protein moiety and yielded a purified protein moiety. Apparent homogeneity was observed for the purified protein with several fractionation techniques. This apparently homogeneous protein dissociates into the constituent proteins in the presence of dodecyl sulphate. The aggregation of proteins is ascribed to their hydrophobic nature. Gel filtration on Bio-Gel A-1.5 m was found to be useful in the separation of the dodecyl sulphate-protein complexes on a preparative scale. Following the removal of the dodecyl sulphate, the renaturation of the thromboplastin protein moiety was unsuccessful. The purified protein moiety with its low lipid content (1 %) is suitable for lipid requirement studies.