Studies on the combined alkaloidal fractions of several species of Erythrina led to the isolation and characterization of two new crystalline compounds containing sulfur. Microanalytical data showed. that one compound had the formula, C20H22NO7S, and the other, C19H21NO7S. They yielded erysovine (C18H21NO3) and erysopine (C17H19NO3), respectively, on hydrolysis. For correlation of names and to designate the sulfur atom present, they were named erysothiovine and erysothiopine. Since the hydrolytic reaction yielded one mole of sulfoacetic acid in each case, erysothiovine and erysothiopine are alkaloidal esters of sulfoacetic acid with erysovine and erysopine. Apparently, they are sulfonic esters. Erysothiovine and erysothiopine are highly active for curare-like paralysis in frogs and they are three to four times more active than the corresponding alkaloids erysovine and erysopine. © 1944, American Chemical Society. All rights reserved.