The study was designed to determine whether ulinastatin can be used as a biological response modifier for the prevention of septicemic shock. Monocytes from heparinized blood of three healthy volunteers were incubated with 0.125 or 1.25 U/ml of ulinastatin, and then endotoxin (Escherichia coli lipopolysaccharide) was added at concentrations of 0.1, 1.0, and 10.0 micrograms/ml. A dose-dependent increase in the production of tumor necrosis factor alpha, interleukin-1 alpha, and interleukin-1 beta was noted after endotoxin stimulation. Production of these cytokines was inhibited by the addition of ulinastatin in a dose-dependent manner. The results indicate that ulinastatin could be useful in the prevention of septicemic shock.