Two procedures to isolate the nuclear matrix from rat liver nuclei were developed. These prevent contamination by nuclear membrane fragments and consist of (A) three consecutive treatments with 1 % Triton X-100 buffer, DNase I digestion and high salt extraction; and (B) a modification of Berezney's method in that isolated nuclei were washed three times with 1 % Triton X-100 buffer in the first step. The nuclear matrices obtained from rat liver nuclei by these procedures consist largely of protein (88.3-88.6 %) with small amounts of RNA (6.5-10.6%), DNA (0.9-4.6%) and phospholipid (0.2-1.2%). Sodium dodecyl sulfate-acrylamide gel electrophoresis of the nuclear matrix proteins gives three major polypeptides with molecular weights of 60, 000-80, 000.