Simultaneous exposure to lead and cadmium is highly probable due to common environmental sources of exposure. Mice were exposed simultaneously for 10 wk to lead and cadmium and effects were observed on accumulation of the metals in tissues and susceptibility to viral challenge. The incidence of viral-induced mortality was enhanced in all groups of mice exposed to lead and suppressed in groups of mice treated with cadmium. Mice exposed to both lead and cadmium had mortality rates intermediate to those observed after exposure to the individual metals. Lead and cadmium residues in tissues of mice exposed to both metals, as compared to residues in tissues of mice which received lead or cadmium only, indicated that interaction occurs in regard to metabolism, storage, and excretion of these elements. Chronic coexposure to lead and cadmium appears to alter certain effects produced by exposure to each metal alone.