Intestinal origin alkaline phosphatase activity (ALP) in plasma was measured by a sensitive immunocapture assay in 104 jaundiced patients--84 with intrahepatic and 20 with post-hepatic jaundice. Increased enzyme activities were observed in those with intrahepatic disease and subnormal values in those with post-hepatic disease. At a discriminant level intestinal origin ALP showed a diagnostic sensitivity of 77% for intrahepatic cholestasis, with a diagnostic accuracy of 75% for its differentiation from post-hepatic jaundice. This diagnostic accuracy is not as good as that derived with other techniques--for example, imaging--and the technique is therefore not recommended as a supplement or replacement.