Serum concentrations of total cholesterol and lipoprotein cholesterol, of apolipoproteins A I and A II and of apolipoprotein A I in lipoprotein particles (Lp A I and Lp A) were determined in 43 patients with multiple myeloma. There were striking alterations in the plasma levels of these analytes relative to normal subjects. We observed a decrease of cholesterol levels in LDL, HDL and HDL3 fractions, and of apolipoproteins A I and A II compared with normal subjects. The HDL2 cholesterol was increased. The decrease of apolipoprotein A II was more prominent than apolipoprotein A I. The decrease of apolipoprotein A I concerns only the A I (Lp A), while the A I (Lp A I) was increased. Most of these modifications were correlated with the monoclonal Ig levels.