Summary Hemocyanin prepared from the circulating fluid of the abalone (Haliotis) was precipitated by ammonium sulphate within the limits found by Alsberg and Clark from Limulus polyphemus. It appears to be a globulin. The sera of rabbits immunized with this preparation of hemocyanin gave positive fixation and precipitin tests. It was toxic for guinea-pigs previously sensitized to this substance but not toxic for normal animals. These experiments support the chemical viewpoint that the chemical makeup of hemocyanin is very different from hemoglobin since the latter is non-antigenic.