Bikunin is a protease inhibitor consisting of a 16 kDa polypeptide and an 8 kDa chondroitin sulphate chain which has an apparent molecular mass of 60-70 kDa upon gel filtration. It is synthesized by hepatocytes and occurs in plasma, both in free form, and in complex with other polypeptides - mainly as the 180 kDa protein inter-a-inhibitor. Bikunin binds to proteases less avidly than other plasma inhibitors, making its role in the blood unclear. However, some observations indicate that bikunin has important functions outside the blood system. To assess its capacity to reach extravascular spaces, we have determined the total concentration of bikunin in plasma (0.17 mg/ml), lymph (31 μg/ml) and bile (0.2 μg/ml). Quantitation after removal of complexed bikunin (inter-α-inhibitor) by acid precipitation showed that the concentration of free bikunin in those fluids was 3, 1.4 and 0.05 μg/ml, respectively. These values yield a lymph/plasma ratio of free bikunin of 0.5, which is higher than expected for a protein of the hydrodynamic size and charge of bikunin. The bile/plasma ratio (0.02), however, is similar to that of other proteins of comparable size. The corresponding values for inter-a-inhibitor, 0.16 and 0.001, respectively, indicate that its capacity to pass through the vascular endothelium is relatively high whereas transfer to bile is restricted. Furthermore, we have found that in a perfusate of an isolated rat liver, the ratio of free to complexed bikunin was 30-40 times higher than in plasma, consistent with previous observations showing that free bikunin is cleared from the blood stream much more rapidly than inter-α-inhibitor.