Abstract: A Preliminory observation and mechanism survey about the effect of immune regulator-BCG on the capacity of resistance against pseudomonas aeruginosa (PA) infection in mice were reported in the present paper. The mice preinfected with BCG (1mg/per mouse iv) were shown to be highly resistant to subsequent challenge with PA (3LD₅₀), and susceptible to lethal effect of rough PA endotoxin. The results of our experiment showed that BCG-induced resistance to PA was related to an enhanced eliminating ability produced by fixed phagocytes of mice and BCG-induced susceptibility to PA endotoxin might be attributed to blockage of reticulo-endothelial system of mice by BCG. The relationship of experimental results was discussed.