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The prevalence of peripheral blood B cells secreting antibodies reacting with the F(ab')2 fragment of monoclonal IgG was studied in five patients with multiple myeloma (MM), nine patients with monoclonal gammopathy of undetermined significance (MGUS) and six healthy controls. An enzyme-linked immunospot assay allowed direct visualization of antibody producing B cells. All patients had B cells producing antibodies to autologous or allogeneic monoclonal IgG. Autoreactive cells were found more frequently than alloreactive cells in seven out of nine patients with MGUS and three out of four patients with MM. The same frequency of alloreactive cells in the patient groups was detected in healthy individuals. These findings show the existence of B cells producing anti-idiotypic antibodies which could be a part of an idiotypic network in monoclonal gammopathies.

An antiserum was developed with specificity for the VkIII subgroup, as evidenced by a precipitin reaction with reference protein Man. The antiserum reacted specifically with 4 of 26 monoclonal IgM, 3 of 57 monoclonal IgG, and 2 of 44 monoclonal IgA immunoglobulins. N-terminal amino acid sequence analysis was performed on kappa chains from 8 of these M-components. All were found to belong to the VKIII subgroup, and J of them had valine in position 2. This high frequency (37%) of valine in position 2 is discussed.

A comparison of murine monoclonal IgG subclass antisera with rabbit polyclonal IgG subclass antisera restriction of the monoclonal antisera and further antigenic heterogeneity within the IgG subclasses of at least two antigenic variants of IgG1, two of IgG2, two of IgG3 and a third subtype of IgG4.