Cytotoxic antibody to a plasma cell tumor antigen was produced in syngeneic BALB mice by immunization with viable or inactivated plasma cell tumors. Antibody with the same specificity was found in the sera of normal BALB and other strains of mice. This natural antibody reacted with an antigen with characteristics indistinguishable from the previously described alloantigen, PC.1, and with viral envelope antigen, χVEA. The incidence of cytotoxic reactivity and the antibody titers reached a peak in normal BALB mice at 3–4 months of age, and were lower in 9–12-month old mice. The sera of germfree mice had lower reactivity; but when the mice were transferred to conventional conditions, their sera soon became as active as those of conventional mice.
A virus common to all plasma cell tumors, which is present in latent form in some normal tissues of BALB and other PC.1 positive strains, is suggested as the cause for the PC.1 antigen and for the appearance of natural antibody to it. The considerable evidence for the close association of a virus with plasma cell tumors is presented.