Monocytes and macrophages from a variety of animal species produce procoagulants upon stimulation with endotoxin in vitro. While C3H/HeJ mice and their cells have exhibited refractoriness to known effects of phenol-water extracted endotoxin, two recent reports indicate that the cells of these mice produce procoagulants in a normal manner in response to endotoxin. This study compares the responsiveness of cells of C3H/HeJ and C3H/HeN mice to phenol-water extracted endotoxin and to disrupted gram-negative cell walls. Phenol-water extracted endotoxin had no mitogenic effect on spleen cells and failed to elicit procoagulant synthesis in exudate macrophages from C3H/HeJ mice. In contrast it was an effective stimulant for spleen cells and exudate macrophages of C3H/HeN mice. Gram-negative cell walls were an effective stimulant for spleen cells and macrophages from both mouse strains. I conclude that exudate macrophages of C3H/HeJ mice do not respond to endotoxin by producing procoagulant.