A protein was solubilized from the myelogenous leukemia cell line K-562 WITh 3 M KCl that specifically inhibited the antibody-dependent, complement-mediated cytolysis of 51Cr-labeled K-562 cells by a monkey antiserum to K-562. When the crude 3 M KCl extract uas fractionated with ammonium sulfate, an eightfold increase in specific activity (U inhibition/mg protein) resulted. This purified fraction migrated as a single protein band after polyacrylamide gel electrophoresis (PGE) with no detectable carbohydrate or lipid. The molecular weight of the denatured protein determined by sodium dodecyl sulfate-PGE was 77,000, similar to that of the native protein (80,000) determined by Sephadex exclusion chromatography. The protein was stable at pH 6-8, with an apparent isoelectric point between pH 5 and 6. In addition to being irreversibly denatured at pH 5 or less, it was unstable at osmolarities below 0.25 M (NaCl). It was denatured at temperatures of 56 degrees C or above. Normal human peripheral blood leukocytes were extracted similarly with 3 M KCl and fractionated with ammonium sulfate. Neither the crude preparation nor any fraction purified as described for the specific antigen inhibited the cytolytic assay, which indicated at least a quantitative lack of the protein on the surfaces of normal leukocytes.