The effects of several factors on the nitrite synthesis were observed in the culture of EMT-6 cells which are originated from mammary adenocarcinoma of Balb/c mouse. The cells were cultured in Dulbecco's Modified Eagle's Medium containing 10% fetal calf serum. Amounts of nitrite in the culture media after 24 and 36 hours of culture were about 2 fold, and 3-fold of those measured after 12 hours respectively. There were very close associations between the amounts of nitrite measured in the culture media. A significant nitrite synthesis by EMT-6 cells occurred when IL-1 was added to the culture medium with other cytokines as IFN gamma or TNF alpha . One of each cytokines were less effective as an inducer of nitrite than the combinations of cytokines. When mercury chloride or cinnamate was added in the culture medium, the nitrite synthesis was dose-dependently decreased by the concentration of these materials. The viability of EMT-6 cells were kept on 95% or above in 36 hours after beginning of culture without any specific additives except cytokines. While after 48 hours it went down to 85% or less. These viability were decreased by the prolongation of culture time (48 hours or more), the addition of TNF alpha to cytokine mixture, and the higher concentrations of mercury chloride or cinnamate to culture medium. Simultaneous addition of the equimolar dose of selenium completely prevented mercurial compounds-induced inhibitions of nitrite syntheses. But the single addition of selenium neither influenced the viability of cells nor the productions of nitrite. These results suggests that the disorder of cell mediated immunity by mercurial compounds could be related to the inhibition of nitric oxide synthesis and selenium decreased the cytotoxicity of mercurial compounds.