In 1993, we reported the presence of an IgM-associated peptide (M(r) 44 kDa; pI 5.45) in all immunoglobulin M (IgM) fractions purified from plasma/serum by various methods. This peptide was absent in Ig fractions of non-IgM isotypes. The N-terminal sequence was determined as being APPSGVRLVGGLH. To gain insight into the nature of this peptide, we further analyzed, using modern proteomic tools, the IgM-associated peptide isolated from cryoglobulins. Mass spectrometry revealed three peptides of different masses: 2203.13 (ELGCGAASGTPSGILYEPPAEK), 1564.83 (KPIWLSQMSCSGR), and 1544.77 (EATLQDCPSGPWGK). Theses sequences together with the already known N-terminal sequence allowed us to identity the IgM-associated peptide as Sp alpha (O43866 in TrEMBL database; CD5 antigen-like). Sp alpha is a member of the scavenger receptor cysteine-rich superfamily of proteins. This family includes the T-and B-cell antigens CD5 and CD6, and several of its members influence immune cell fate. Our finding may have important implications in the understanding of the homeostasis of IgM antibodies.