A counterimmunoelectrophoresis (CIE) test for the detection of liver-kidney microsome specific antibodies in human sera is described. By testing different subcellular preparations the LKM antigen was found in the membranes of the smooth endoplasmic reticulum subfraction. The antigen was sensitive to trypsin digestion and behaved as an anionic protein in the experimental conditions used in the test. All sera positive for LKM in immunofluorescence gave a precipitin line of identity while none of the control sera gave a positive reaction. The CIE titers ranged between neat and 1/4096. A significant correlation was observed between the LKM titers obtained in immunofluorescence and those obtained in CIE. Moreover, by absorption experiments, it was concluded that the antigen preparation reactive in CIE was able to abolish the immunofluorescence pattern of LKM positive sera on rat liver and kidney sections. The LKM target antigen, although previously considered a structural protein of microsomal membranes, was shown to solubilize spontaneously during the isolation of microsomal membranes. Counterimmunoelectrophoresis appears to be an appropriate test for anti-LKM antibodies in human sera.