The crystal structure of the haematopoietic cell kinase Hck has been determined at 2.6/2.9 Angstrom resolution. Inhibition of enzymatic activity is a consequence of intramolecular interactions of the enzyme’s Src-homology domains SH2 and SH3, with concomitant displacement of elements of the catalytic domain. The conformation of the active site has similarities with that of Inactive cyclln-dependent protein kinases.