An unusual paraprotein of very rapid electrophoretic mobility and γA antigenicity has been found to have its L chain antigenicity obscured or hidden, although associated Bence Jones protein in the urine exhibited easily demonstrable λ specificity. The intact protein was not typable with anti-κ or anti λ sera and when injected into rabbits failed to provoke either anti-κ or anti λ response. Mercaptoethanol reduction of the protein revealed a normal proportion of L chains, which were readily shown to have λ specificity and which had an acid urea starch gel electrophoretic mobility identical to that of the reduced Bence Jones protein. The isolated L chains provoked a λ-specific response when injected into rabbits. The reduced and alkylated paraprotein (its L and H chains not yet separated by acid sephadex filtration) was readily typable as λ-specific when the same typing sera was employed which failed entirely to react with the unreduced whole protein. Five other γA myeloma proteins showed enhanced precipitation with κ and λ typing sera after mercaptoethanol reduction of the paraproteins. It is concluded that the atypical antigenic properties displayed by the paraprotein described in this report probably reflect structural features producing steric hindrance against reaction with homologous antibodies.

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