The capacities of bacterial DNA, extracted from Salmonella typhimurium, and lipopolysaccharide (LPS), extracted from Salmonella minnesota, to activate mouse peritoneal macrophages in vitro were compared. Activation was assessed by estimating e levels of 3 cytokines, IL-10, IL-12, and $IL-1{eta}$, at time intervals of 3, 6, 9, and 24 h after addition of LPS and/or DNA to macrophage cultures. Cytokine levels in culture supernatants were determined by enzyme-linked immunosorbent assay (ELISA) and cytokine mRNA levels were estimated based on band intensity in cultured cells by reverse transcriptase-polymerase chain reaction (RT-PCR). Results obtained demonstrated the ability of DNA and LPS to elicit increased production of all 3 cytokines as compared to controls. In the amount tested, LPS appeared to be a more potent inducer of IL-12, and $IL-1{eta}$, whereas DNA induced higher levels of IL-10. DNA and LPS, used in combination, exhibited neither an additive nor a synergistic effect. Rather, an antagonist effect appeared to occur. RT-PCR results correlated well with ELISA.