T lymphocytes play a critical role in the development of murine collagen-induced arthritis (CIA), a syndrome which shares many features with rheumatoid arthritis. In susceptible mouse strains, immunization with type II collagen (cII) results in chronic inflammation with progressive joint destruction. To examine T cell recognition of cII we have isolated T cell hybridomas specific for a cII peptide fragment, cII260-270 (IAGFKGEQGPK). We assessed the importance of particular amino acid residues to formation of the MHC Class II-peptide complex and interaction of this complex with TCRs. Our results indicate that critical residues are concentrated in the N-terminal half of the peptide, with Lys264 and Glu266 having the greatest influence. Replacement of these residues with alanine resulted in loss of detectable activity. Three other residues, Ile260, Gly262, and Phe263, are also important to T cell stimulation because alanine substitution substantially decreased peptide activity. Truncation analyses supported the conclusion drawn from Ala substitutions that C-terminal residues were dispensable. Other alterations in peptide structure resulted in dramatic increases in stimulatory capacity. For example, replacement of either Gly265 or Gly268 with alanine resulted in a 10-fold increase in potency. The addition of 2-5 residues to the N-terminus of the peptide decreased the dose required for maximal T cell stimulation by > 100-fold. The T cell hybridomas exhibited remarkable similarity in their peptide recognition profiles although they express different TCR V beta elements.