A new thermostable caseinolytic serine protease was purified from the latex of Euphorbia heterophylla L. to electrophoretic homogeneity by a procedure involving successive steps of pretreatment of the latex, PEG fractionation, CM-cellulose chromatography and DEAE-cellulose chromatography. The purified protease was found to be a monomeric protein of molecular weight 77.2 kDa. It exhibited caseinolytic activity with hyperbolic azocasein saturation with Vmax and Km values of 0.11 units.mL(-1) and 0.55 mg.mL(-1) respectively. Specific inhibitory studies revealed the enzyme to be a serine protease. The protease was characterized by pH optimum of 8.0 and high thermostability with T1/2 of 75°C. Based on the results of peptide mass fingerprinting analysis, the protease was shown to be a new protein not characterized earlier.