Summary: No evidence of specific complement fixation could be demonstrated when immune sera, produced in rabbits with pure vaccinia brain-virus, were tested against antigens prepared from the lesions of cutaneous vaccinia or against brain-virus antigens. No specific complement fixation occurred when immune sera, produced with cutaneous vaccinia virus, were tested against antigens prepared from brain-virus. Although the latter sera fixed complement in the presence of antigens prepared from cutaneous vaccinia pulp, an almost equivalent degree of complement fixation was observed in the presence of antigens prepared from bacteria cultured from this pulp. Antisera produced against bacteria, harvested from the pulp of cutaneous vaccinia lesions, fixed complement, in the presence of antigens prepared from cutaneous vaccinia lesions. No evidence for the presence of specific precipitins in the two kinds of vaccinia immune sera could be elicited. Precipitates, which did occur in certain cases, were shown to be due to unspecific reactions. Virulicidal antibodies were easily demonstrable in all the immune sera examined for this property.