Plasmid-encoded class C beta-lactamases, including CMY-1 and CMY-10, hydrolyze the lactam bonds of beta-lactam antibiotics, inducing therapeutic failure and a lack of eradication of clinical isolates by third-generation cephalosporins or cephamycins. Therefore, the enzymes are potential targets for developing agents against pathogens isolated from patients suffering from wound infection, urinary tract infection or pneumonia. CMY-1 and CMY-10 were purified and crystallized at 298 K. X-ray diffraction data from CMY-1 and CMY-10 crystals have been collected to 2.5 and 1.5 A resolution, respectively, using synchrotron radiation. The crystals of the two proteins are isomorphous and belong to the primitive monoclinic space group P2(1).