The possible effect of lithium chloride, a compound which reduces the incidence of infection in cancer patients, was investigated on murine melanoma. C57 BL syngeneic mice were inoculated i.p. with B16 melanoma cells. The animals were divided into 4 groups, receiving daily i.p. treatment with saline--group 1, controls, lithium chloride--group 2, bleomycin and vinblastine--group 3, and lithium chloride with bleomycin and vinblastine--group 4. Animals in group 4 had a significant delay in tumour appearance, a higher degree of tumour necrosis, and a longer survival rate. In addition a significant reduction of serum lithium concentration was noted in animals of this group in comparison with animals in group 2, treated with lithium chloride alone. There was no lithium-induced leukocytosis.