An in vitro complex formation between cholesterol and human α 1-proteinase inhibitor (α 1-antitrypsin, α 1-Pi) has been described. Hydrophobie bile acids were studied for a similar interaction using lithocholic acid (LC) as a prototype of a hydrophobic acid. At a molar ratio of 5:1, LC induced conformational changes of α 1-Pi reflected in an abnormal gel-electrophoretic appearance, loss of anodal immunoreactivity on crossed immunoelectrophoresis, exposition of new antigenic determinant(s) on immunodiffusion, and loss of antiproteinase activity. After 6 h incubation, LC and α 1-Pi form a complex of approximately 200 kDa molecular mass seen following gel-filtration. After prolonged (24 h) interaction a series of large α 1-Pi polymers were seen on SDS-PAGE under reducing conditions followed by Western blotting. Glycolitho-, sulfolitho-, deoxycholic and 3-β-hydroxy-5-cholenoic acids induced similar but less pronounced changes of α 1-Pi, whereas transferrin remained unaffected. Hydrophilic acids lacked effect on α 1-Pi. The results are compatible with a specific, irreversible interaction of α 1-Pi with hydrophobic bile acids affecting its physical and proteinase inhibitory properties. The cholestatic potency of the hydrophobic acids studied and their ability to induce α 1-Pi polymerization may be important in cholestatic conditions.