Three human serums containing artificially produced blocking antibody against low ragweed allergen were studied for the possibility of relating blocking antibody to electrophoretically definable components. An adaptation of the qualitative passive transfer test to quantitative interpretation is described, methods and procedures are given and uncertainties and possible errors due to lack of precision and accuracy are presented and discussed. At least 65 per cent, but probably more, if not all of the blocking antibody is attributable to gamma globulin. However, no rise of gamma globulin, either its absolute amount or its relative percentage value, paralleled the appearance of blocking antibody. Blocking antibody is not contained in albumin or in alpha-1 globulin. Blocking effect could not be ascertained unequivocally as being connected with alpha-2 or with beta globulin and sizable participation of these two latter electrophoretic components appears improbable. Blocking antibody and sensitizing antibody appear to be chemically different entities.