A new method utilizing glass plates covered with a thin layer of fat was employed in the study of lipoid degradation especially that of beef tallow by pathogenic leptospirae. The tested fats were attacked most vigorously by virulent leptospirae belonging to serotypes L. icterohaemorrhagiae, L. canicola and L. pomona, to a lesser extent by types grippotyphosa and hebdomadis. No demonstrable lipoid cleavage was evidenced by virulent strains hyos, sejroe and saxkoebing, or by avirulent strains subjected to protracted laboratory culture. Lipase contained in the leptospirae causes the fats to give off oil droplets probably drops of fatty acid with a high content of C atoms, while releasing toxic, hemolytic decomposition products into the soapy-smelling nutrient. The pathological importance of fat cleavage by leptospirae ought to involve the fact that serotypes which attack lipoid most intensely are usually capable of causing jaundice and hemorrhages in various tissues.