Cadmium-induced metallothionein may have a paradoxical role in the pathogenesis of cadmium toxicity. Mice are protected from cadmium-induced testicular necrosis by metallothionein, but in this experiment it was shown that cadmium-metallothionein complex injected into mice results in higher cadmium levels in renal cortex and more severe renal tubular cell injury than a comparable dose of cadmium chloride administered by the same route. Also, an injection of 1.1 mg/kg of body weight or more of cadmium bound to metallothionein was fatal to male CGA mice within seven days whereas five times this dose of cadmium administered as cadmium chloride was needed to produce a similar effect. A reason for the difference in renal effects of these two forms of cadmium has not been determined but may be related to differences in absorption or transport by renal tubular lining cells.