Macrophages isolated from the anterior kidney of juvenile chinook salmon Oncorhynchus tshawytscha in 96-well microtiter plates were exposed for 72 h to 0, 10⁵, or 10⁶ live or heat-killed Renibacterium salmoninarum cells per well or to 0, 0.1, 1.0, or 10 μg/mL of R. salmoninarum soluble proteins. After treatment, the bactericidal activity of the macrophages against Aeromonas salmonicida was determined by a colorimetric assay based on the reduction of the tetrazolium dye MTT to formazan by viable bacteria. The MTT assay was modified to allow estimation of the percentage of bacteria killed by reference to a standard curve relating the number of bacteria added to microtiter wells to absorbance by formazan at 600 nm. The live and heatkilled R. salmoninarum treatments significantly (P < 0.001) increased killing of A. salmonicida by chinook salmon macrophages. In each of the five trials, significantly (P < 0.05) greater increases in killing occurred after exposure to 10⁵ R. salmoninarum cells than to 10⁶ R. salmoninarum cells per well. In contrast, treatment of macrophages with 10 ug/mL R. salmoninarum soluble proteins significantly (P < 0.001) decreased killing of A. salmonicida, but treatment with lower doses did not. These results show that the bactericidal activity of chinook salmon macrophages is stimulated by exposure to R. salmoninarum cells at lower dose levels but inhibited by exposure to R. salmoninarum cells or soluble proteins at higher dose levels.