Sera of fourteen rabbits injected with alum-precipitated recrystallized ovaltumin, containing 0.046 to 0.604 mg. of precipitable antibody nitrogen per ml. (average 0.299 mg.), passively sensitized human skin, while the sera of nine rabbits injected with dissolved recrystallized ovalbumin, containing from less than 0.05 to 0.420 of antibody nitrogen per ml. (average 0.176 mg. or less), were inactive in human skin. The skin-sensitizing activity of the sera bore no relation to the precipitin content. Removal of 68 to 90 per cent of the precipitin nitrogen by a single addition of antigen did not affect the activity of the sera in sensitizing human skin. Removal of all precipitable antibody nitrogen in one serum by a single addition of antigen removed the skin-sensitizing activity. The "univalent" antibody remaining after complete removal of precipitin by fractional addition of antigen showed the same activity in passive sensitization of human skin as the original serum.