Understanding the structural features of naturally processed peptides found within the major histocompatibility complex (MHC) class II peptide binding groove from disease-associated MHC molecules may provide insights into the nature of potential disease-related antigens. Class II MHC/peptide complexes were purified by immunoaffinity from transformed B cell lines homozygous for DRB1*0404 (an allele associated with rheumatoid arthritis) and *0402 (a closely related allele not associated with this disease). Peptides were eluted at acidic pH, fractionated by reversed phase HPLC, and analyzed by capillary electrophoresis. Those fractions containing a single dominant peptide were sequenced by automated Edman degradation and tandem mass spectrometry. The predominant peptide species identified came from non-polymorphic regions of the HLA class I molecules expressed by each cell line. Peptides from DRB1*0404 were found to be nested clusters derived from positions 26-43 of the HLA-B and -C alpha-chain. DRB1*0402 contained as the predominant peptide species a nested cluster from positions 129-145 of the HLA-B alpha-chain. The primary structure of the class I derived peptides was consistent with that seen by peptides exhibiting promiscuous DR binding behavior. Processing of MHC-derived peptides by MHC class II molecules is a common occurrence in the transformed B cell lines analyzed.