Small amts. of other protein constituents besides hemocyanin are found in lobster-serum. The "class-specific" heteroagglutinins of lobster-serum are found to reside in a component that is obtained free of hemocyanin by isoelectric precipitation at pH 4.8-5. Electro-phoresis of this "pale precipitate" reveals the presence of 2 components, of which the more slowly migrating one bears the heteroagglutinating activity. The 10 separate "class-specific" heteroagglutinins are thus evidently represented by a single electrophoretic component. There is some indication that fibrinogen obtained from the lobster plasma may also act as heteroagglutinin. Antibodies produced in rabbits against purified hemocyanin also react with the slow electrophoretic component (heteroagglutinin) of the pale precipitate and with fibrinogen. Absorption tests with antisera vs. whole lobster-serum fail to reveal the presence of any specific antigenic groups other than those of the hemocyanin. The other blood proteins are, then, evidently serologically equivalent to hemocyanin.