An anti-neoplastic protein was isolated from the endosperm of the seeds of Caesalpinia gilliesii by extraction with water, dialysis and precipitation by ammonium sulfate or acidification. The precipitated protein mixture was separated by column chromatography into three principal proteins, one of which, termed cesalin, inhibited the growth of Walker 256 carcinosarcoma. There is an associated carbohydrate with the cesalin that can be largely removed by chromatography on hydroxyl apatite; the remaining carbohydrate (about 0.3%) is a hexosan. Cesalin, molecular weight 110,000, migrates as a single component by polyacrylamide gel electrophoresis at pH 8.3, but in a denaturing system containing sodium dodecyl sulfate three bands were observed. The correspond to protein sub-units of approximately 30,000 daltons. Anti-tumor tests in rats showed 70-80% inhibition of Walker 256 growth at a dose of 80 microgram/kg/day of cesalin.