Nuclear matrix preparations of rat hepatoma 27 differed from those of rat liver tissue in predominance of polypeptides with molecular weights of about 220, 160, 100, 45 and 31 kilodaltons. An inhibitor of serine proteinases phenylmethylsulfonyl fluoride did not alter practically the yield or protein pattern of rat liver or hepatoma 27 nuclear matrices. At the same time, thiol reagents mersalyl and 5,5-dithio-bis(2-nitro-benzoic acid) increased 2-3-fold the yield of both rat liver and hepatoma 27 nuclear matrices elevating mainly the content of polypeptides of 34-45 kilodaltons molecular weights.