An X-ray structure of L-asparaginase from Erwinia chrysanthemi (ErA) has been refined at 1 A resolution to an R factor of below 0.1, using data collected on a synchrotron source. With four molecules of the enzyme consisting of 327 amino acids each, this crystal contains one of the largest asymmetric units of a protein refined to date at atomic resolution. Previously, structures of ErA and of related enzymes from other bacterial sources have been refined at resolutions not exceeding 1.7 A; thus, the present structure represents a very significant improvement in the quality of the available models of these proteins and should provide a good basis for future studies of the conformational variability of proteins, identification of subtle conformational features and corroboration of the stereochemical libraries, amongst other things. L-Asparaginases, which are enzymes that catalyze the hydrolysis of L-asparagine to aspartic acid, have been used for over 30 y as therapeutic agents in the treatment of acute childhood lymphoblastic leukemia, although the details of the enzymatic reaction and substrate specificity have not yet been completely elucidated. This atomic resolution structure is a step in that direction.