The anion gap was evaluated in 26 consecutive patients with multiple myeloma (15 IgG and 11 IgA). The average union gap was found reduced only in IgG myeloma. Instead, in the subjects with IgA myeloma it resulted as increased. The partial differences between our results and those of other authors are discussed. The finding of correlations between the anion gaps and paraproteinemic concentrations (direct in IgA myeloma, inverse in IgG myeloma) gives further support to the conception that the charge exhibited in the serum by IgG and IgA paraproteins is, respectively, positive and negative. It is suggested that a 'normalization' of the anion gap during chemotherapy may be a useful tool to judge the efficacy of the treatment of multiple myeloma.