Introduction: Metabolic acidosis frequently complicates sepsis and septic shock and may be deleterious to cellular function. Different types of metabolic acidosis, (e.g. hyperchloremic, lactic acidosis) have been associated with different effects on the immune response but direct comparative studies are lacking. Methods: Murine macrophage-like RAW 264.7 cells were cultured in complete media with lactic acid or HCl to adjust the pH between 6.5 and 7.4, and then stimulated with lipopolysaccharide (LPS, E. coli 0111:B4; 10 ng/ml). Nitric oxide (NO), interleukin (IL)-6 and IL-10 levels were measured in the supernatants. RNA was extracted from the cell pellets and RT-PCR was performed for amplifying corresponding mediators. Gel shift assay was also performed to assess the NF-B DNA binding. Results: Increasing concentrations of acid caused increasing acidification of the media. Trypan blue exclusion and LDH release demonstrated that acidosis did not reduce cell viability. HCl significantly increased LPS-induced NO release and NF-B DNA binding at pH 7.0 but not at 6.5. Both IL-6 and IL-10 expression (both RNA and protein) were reduced with HCl-induced acidification but IL-10 was reduced much more than IL-6 at low pH. By contrast lactic acid significantly decreased LPS-induced NO, IL-6, and IL-10 expression in a dose-dependent manner. Lactic acid also inhibited LPS-induced NF-B DNA binding. Conclusion: Two common forms of metabolic acidosis (hyperchloremic and lactic) are associated with dramatically different patterns of immune response in LPS-stimulated RAW 264.7 cells. HCl is essentially pro-inflammatory as assessed by NO release, IL-6:IL-10 ratios and NF-B DNA binding. By contrast lactic acidosis is anti-inflammatory.