The two metal sites in cadmium substituted beta -lactamase from Bacillus cereus 569/H9 have been studied by NMR spectroscopy (H-1, N-15, and Cd-113) and PAC spectroscopy (Cd-111m). Distinct NMR signals from the backbone amides are identified for the apoenzyme and the mononuclear and binuclear cadmium enzymes. For the binuclear cadmium enzyme, two Cd-113 NMR signals (142 and 262 ppm) and two Cd-111m PAC nuclear quadrupole interactions are observed. Two nuclear quadrupole interactions are also observed, with approximately equal occupancy, in the PAC spectra at cadmium/enzyme ratios <1; these are different from those derived for the binuclear cadmium enzyme, demonstrating interaction between the two metal ion binding sites. In contrast to the observation from PAC spectroscopy, only one Cd-113 NMR signal (176 ppm) is observed at cadmium/enzyme ratios < 1. The titration of the metal site imidazole, (N)H proton signals as a function of cadmium ion-to-enzyme ratio shows that signals characteristic for the binuclear cadmium enzyme appear when the cadmium ion-to-enzyme ratio is between 1 and 2, whereas no signals are observed at stoichiometries less than 1. The simplest explanation consistent with all data is that, at cadmium/enzyme ratios < 1, the single Cd(H) is undergoing exchange between the two metal sites on the,enzyme. This exchange must be fast on the Cd-113 NMR time scale and slow on the Cd-111m PAC time scale and must thus occur in a time regime between 0.1 and 10 s.