SummaryStarting from the observation of a decreased anion gap in myelomatous patients, the authors try to define a general law relating anion gap to serum levels of immunoglobulins. A correlation is shown between anion gap and difference between anionic and cationic immunoglobulins, in twenty-eight patients without obvious cause of abnormal anion gap. The linear regression equation of anion gap (mEq/1) on IgG-(IgA + IgM) (m mol/1) is y = 7.03–50.44 x (r = 0.44, p<0.02).This relationship applies in particular to patients with myeloma.