Abstract: The invention provides a peptide suitable for the prophylaxis, treatment, diagnosis or monitoring of arthritis. The peptide contains a contiguous sequence element of 9 amino acid residues (nonapeptide), having the amino acid sequence: X1-X2-X3-X4-X5-X6-X7-X8-X9. In the sequence, X2, X6 and X7 are any amino acid; X1 is any amino acid except K, H, R, E, D; X3 is S or T; X4 is one of F, L, I, V, A, G, C, P; X5 is one of A, G, C, P, S, N, T, V; X8 is one of V, L, I, M; and X9 is E or D. The peptide is also a part of a mammalian cartilage, joint or arthritis-related protein such as collagen or a cartilage protein, and is between 9-20 amino acids in length. Furthermore, the peptide is recognised by freshly isolated T cells from rats in which Adjuvant Arthritis is induced and/or is recognised in vitro by T cell clone A2b.