We have analyzed HLA class II alleles in a group of 153 Czech children with rheumatoid arthritis by PCR and hybridization with oligonucleotide probes. When we try to find a common sequence for all DRB1 alleles involved in juvenile and adult arthritis, we can notice hydrophobic amino acid at position 74, which is present in all these alleles, but not in nonsusceptible alleles, where is the hydrophilic amino acid at position 74. In our model, we speculate that the hydrophilic amino acid at position 74 creates a such kind of epitope which is not suitable for rheumatoid-associated peptides or T cells, and only hydrophobic amino acid can permit binding of these peptides or recognition by certain T cells. Analyses of the DPB1 sequences have shown that alleles which have a negatively charged amino acid at position 69, are more frequent in pauciarticular patients while those with a positively charged amino acid are more frequent in polyarticular patients. A positively charged amino acid at position 69 might present the same rheumatoid associated peptide as susceptible DRB1 alleles. The presence of more rheumatoid-associated peptide on the cell surface may cause conversion to more severe polyarticular forms. A negatively charged amino acid at position 69 could not present this peptide and a low concentration of the peptide on the cell surface presented just by DRB1 molecules keeps disease in a relatively benign condition of pauciarticular forms.