Serum levels of seven specific proteins mostly acute phase reactants (APR) have been studied (transferrin, alpha 2-macroglobulin, alpha 1-antitrypsin, haptoglobin, C3, ceruloplasmin, orosomucoid) in 14 healthy subjects and in 55 patients with multiple myeloma. The alpha 2-macroglobulin and transferrin are significantly decreased in the myeloma group compared with healthy controls whereas the remaining proteins under study are elevated, significantly only orosomucoid and ceruloplasmin. The APR in the IgG3 myelomas seem to show differences from those of the IgG1 and IgG2 subclasses the levels of the negatively changed proteins being lower, whereas the positive APR higher in the IgG3 myeloma. In the IgA myeloma, however, decreased levels of haptoglobin and alpha 1-antitrypsin have been found in spite of being positive APR. Transferrin and haptoglobin serum level can be included as a new parameter in regression equations for calculation of IgA myeloma cell mass.