Pseudomonas aeruginosa (P. aeruginosa) is a gram-negative bacterium that causes a variety of diseases in compromised hosts. Bacterial endotoxins such as lipopolysaccharide (LPS) are the major outer surface membrane components that are present in almost all gram-negative bacteria and act as extremely strong stimulators of innate immunity and inflammation of the airway. This study was undertaken to determine the effect of combined administration of Gentamicin (GENT) as an antibiotic and Dexamethasone (DEXA) as an anti-inflammatory drug on some immunological and histological parameters. After determination of LD50 of P. aeruginosa, mice groups were injected with DEXA, GENT and lipopolysaccharide alone or in combination. Lipopolysaccharide single injection caused a significant increase of total leukocyte count, lymphocytes, neutrophils and levels of IgM and IgG. DEXA induced an increase of neutrophilia and lymphopenia. Immunological examination demonstrated that combined treatment has a significant effect of decreasing lymphocytes and IgG levels than single treatment does. Histological examination demonstrated that the inflammation of thymus, spleen, lymph node and liver decreases in mice that received combined treatment than those that received individual treatment. Concurrent administration of DEXA and GENT has a great effect on protecting organs against damage in case of endotoxemia.