Major histocompatibility complex (MHC) class II molecules are key players in antigen-specific CD4(+) T cell stimulation, despite their lack of discrimination between "self" and foreign antigens. The susceptibility of many individuals to autoimmune diseases is directly attributed to this lack of specificity, as well as to the expression of subclasses of MHC class II molecules. Increased susceptibility to the autoimmune disease rheumatoid arthritis (RA) has been attributed to the expression of the MHC class II alleles HLA-DR1 and HLA-DR4. To define the structural requirements of the HLA-DR4 protein in the autoimmune response of RA, we have crystallized HLA-DR4 with the immunodominant peptide from human collagen II, covalently linked to the N-terminus of the beta-chain [HLA-DR4/hCII(257-273)]. Crystallization time, crystal size, and reproducibility were greatly improved by macroseeding into microdialysis buttons.