A model is proposed for the structure of an inhibited derivative of an enzyme which hydrolyses proteins. It is based on a map of the electron density distribution at 2 Å resolution and interpreted in terms of a previously reported sequence of 241 amino-acids. The map has been derived from a Fourier synthesis of 24,500 terms and represents two crystallographically independent molecules, each of molecular weight 25,000, which are nearly identical in their tertiary structure. The enzyme is composed almost entirely of extended polypeptide chains. A chemical marker unambiguously defines the position of the active centre. The position and orientation of the residues known to be important in catalysis are clearly seen. The mechanism by which the inactive precursor is converted into an active enzyme is revealed.