The crystal and molecular structure of cephaloridine hydrochloride monohydrate (Fig. 1), an antibiotic inhibiting the synthesis of bacterial {A figure is presented} cell walls in a fashion similar to that of the penicillins, has been determined by three-dimensional, single crystal X-ray diffraction methods. The central part of the molecule is found to be similar to that of cephalosporin C. © 1969.