In this study albino mice were used to test the beneficial effect of bacterial lysates vaccination against lethal dose of Pseudomonas aeroginosa. Escherichia coli lysate, Pseudomonas aeroginosa lysate, Staphylococcus aureus lysate and mixed bacterial lysates were administered through the oral and the intranasal routes, both in the presence and absence of Freund's adjuvant versus a placebo. Pseudomonas aeroginosa fifty percent lethal dose ( LD50 ) was injected intraperitonealy following intranasal and oral vaccination. The placebo and the four bacterial lysates were also used in association with Freund's adjuvant. The results of the LD50 in intranasal vaccinated groups were 50%, 37.5%, 100%, 0% and 12.5%, and those with Freund's adjuvant were 25%, 25% , 12.5%, 62.5%, 0% and 0% respectively. The results of LD50 in oral vaccination were 50%, 25%, 62.5% 0% and 37.5%, and those treated with Freund's adjuvants were 12.5%, 12.5 %, 12.5%, 37.5 %, 0% and 25% respectively. The bacterial lysates vaccinated groups were studied for the total body weight (T), liver (L), spleen (S), thymus (Th) weights and the L+S+Th/T ratio. Besides, the peripheral blood and the peritoneal fluid total and differential leucocytic counts were determined and the bone marrow lymphocyte percentage. The serum immunoglobulins G and M were assessed using the immundiffusion plates. Our conclusion is: Bacterial lysates can play an important role as immunomodulators when used by oral or intranasal routes.