A curude cornin extracted from rat liver microsomes and the chemical properties of the purified cornin (MF1) were investigated. 1. Crude rat liver microsomal cornin (RLMC) inhibits the growth of L-929 cells. MF1 is purified about 4 fold from RLMC by gel filtration through Sephadex G-50 column. 2. The concentration of MFI that inhibits the growth of L cells to 50% is 270 μg/ml and this specific activity is the highest of the cornin fractions reported up to date. 3. The content of acidic amino acids is more than 23% but that of histidine and sulfurcontaining amino acids is very low in MFl. 4. MFl shows the maximum ultraviolet absorption at 276nm and its main substance is a kind of protein, and its inhibitory effect on the cell growth diminishes by protease treatment. 5. MFl is a single peak both by polyacrylamide gel electrophoresis and by thin layer gel filtration and the molecular weight is calculated about 26,000. 6. MFl is separated in two bands by SDS-polyacrilamide gel electrophoresis and the molecular weight is estimated to be 11,000 and 15,000 respectively.