Using fyn PTK as a template, a series of phosphopeptides 1-11 spanning in length from 1-14 amino acids was prepared. Kinetic evaluation of 1-11 suggest that CD45 does not have a strong preference for its N- or Cterminal amino acids and that extended phosphopeptides are not required for efficient substrate turnover.CD45 is a protein tyrosine phosphatase expressed on the surface of T-cells and other nucleated hematopoietic cells. 1 The enzyme catalyzes the hydrolysis of the O-P bond in O-phosphorylated tyrosine residues of certain protein tyrosine kinases (PTKs). Ib Dephosphorylation of these tyrosine kinases by CD45 results in the up-regulation of their catalytic activity, initiating a cascade of intracellular events leading to T-cell activation. 2 CD45 therefore represents a therapeutic target for certain autoimmune and chronic inflammatory diseases characterized by aberrant T-cell activation.CD45 interacts with members of the src-family PTKs, most notably c-fyn and the T-cell specific kinase lck.lb, 3 The secondary amino acid sequence of the src PTKs surrounding the site of tyrosine dephosphorylation reveals a high degree of sequence homology (Figure 1). 3 Using fyn PTK as a template, a series of N-acetyl phosphopeptide amides 1-11 spanning in length from 1-14 amino acids was synthesized and evaluated as substrates for CD45. Our intent was to ascertain the smallest phosphopeptide sequence required by the enzyme for substrate recognition and to use this information as a starting point for the rational design of peptide-based inhibitors.