AbstractThe specificity of Kirkman‐Robbins hepatoma and hamster liver non‐histone chromatin proteins has been studied by comparing polypeptide patterns in polyacrylamide gel electrophoresis and by their immunological activity in the complement fixation test. Non‐histone proteins were separated from DNA with a polyethylene glycol‐dextran mixture and fractionated by hydroxylapatite chromatography into three classes named NHCP1, NHCP2, and NHCP3. Electrophoretic analysis indicated that among the non‐histone proteins of Kirkman‐Robbins hepatoma and hamster liver differences mainly of a quantitative nature can be observed. However, the polypeptides with molecular weight 25 000, 31 000, 36 000, 73 000 in NHCP1; 20 000, 40 000 in NHCP2 and 20 000, 32 000, 38 000, 44 000, 75 000, 80 000 in NHCP3 were found to be specific for hepatoma chromatin. Application of antibodies against NHCP1, NHCP2 and dehistonized chromatin of Kirkman‐Robbins hepatoma revealed that the highest specificity of NHCP2 eluted from hydroxylapatite with 100 mMphosphate buffer at pH 6·8. The NHCP1 of hepatoma shares some common antigenic determinants with analogous proteins of liver. On the other hand non‐histone proteins specific for heptoma dehistonized chromatin can be localized in the NHCP3 and partially in the