Salts of Cd, Rb, Hg and Ni given orally to mice increased encephalomyocarditis virus-induced mortality rates. Although Pb was the least toxic of the 4 metals, it enhanced the mortality the most. Concentrations of HgCl2 as low as 0.01 ppm intensified the mortality; a minimal concentration that resulted in no effect was not ascertained. Zn deficiency caused pronounced retardation of growth in young mice, but it did not influence mortality due to encephalomyocarditis virus.