A large series of NZB plasmacytomas was studied to determine the classes of immunoglobulin produced. The time course for plasmacytoma development and the frequency of immunoglobulin classes produced differed significantly from tumors produced in BALB/c mice. Unique immunoglobulins carrying markers of both IgG3 and IgA immunoglobulin classes were detected. SAMM 368 is a BALB/c plasmacytoma secreting two classes of immunoglobulin - IgG2b and IgA - which do not share idiotypes. The IgG2b molecule does not express the appropriate allotype markers for the class. Studies of this protein suggest that allotype markers for IgG2b are expressed by the CH3 domain.