Summary: The precipitin reactions of highly purified conalbumin and six-times recrystallized ovalbumin with their homologous horse- and rabbit-antisera have been studied. Traces of impurity present in both antigens were easily detectable by immunological methods. Although the ovalbumin showed two components electrophoretically these were indistinguishable immunologically. A small amount of highly antigenic impurity may appreciably distort the quantitative curve due to the major component.