1. We have studied the plasma lipoprotein abnormalities in obstructive jaundice to test the hypothesis that the abnormalities would correlate with plasma lecithin--cholesterol acyltransferase activity. 2. Very-low-density lipoproteins (VLDL) were normal in composition and had a normal pre-beta electrophoretic mobility when lecithin--cholesterol acyltransferase activity was high. When it was low VLDL had abnormal composition and ran in the beta position. 3. With high lecithin--cholesterol acyltransferase activity, low-density lipoprotein (LDL) was normal and cholestatic LDL (LP-X) was not found. With low lecithin--cholesterol acyltransferase activity up to three LDL fractions were found: (i) large triglyceride-rich particles, (ii) LP-X and (iii) a triglyceride-rich cholesteryl ester-poor particle of normal dimensions. 4. High-density lipoprotein (HDL) concentrations correlated with lecithin--cholesterol acyltransferase activity and HDL were normal in composition and electron-microscopic appearance when the activity was high. When lecithin--cholesterol acyltransferase activity was low HDL was abnormal in composition and 'stacked discs' were seen on electron microscopy as well as normal spherical particles. 5. These results suggest that low lecithin-cholesterol acyltransferase activity may cause at least some of the lipoprotein changes of obstructive jaundice.