Objective: To test the hypothesis that acute phase reactants, such as alpha 1-antitrypsin and alpha 1-acid glycoprotein, could protect mammalian cells from further damage.Methods: Human dermal fibroblasts (5 × 104) were cultured with DMEM plus 10％ FBS at 37℃ in a 5％ CO2incubator. Different doses of LPS (lipopolysaccharide) and/or acute phase reactants were added. After 24 hours, the cultured supatant was aspirated, the cells were washed, fixed and stained by methylene blue. The unbound stain was washed off. The stained cells were solubilized in 0.1 mi of 1％ Triton X-100. The absorbance of each well was measured using an ELISA spectrophotometer. The concentration of LPS which decreased the absorbance to 70％ of the control LPS-free ) cultures was defined as LD30.Results: In order to achieve LD30 in the presence of acute phase proteins, it was necessary to alter the LPS concentrations. The LD30 of LPS treated with 0, 0.5, 2, 10mg/ml antitrypsin and 0, 0.5, 2, 10 mg/ml glycoprotein was 5.4, 6.5, 7.6, 14.2 mg/ml and 5.2, 5.9, 6.9, 10.5mg/ml, respectively. Statistically, with the treatment of more than 2 mg/ml antitrypsin or glycoprotein, LD30increased significantly.Conclusions: Our data show that fibroblasts are susceptible to the direct toxicity of LPS. Alpha 1-antitrypsin and alpha 1-acid glycoprotein can reduce the toxicity and/or increase the tolerance of mammalian cells to LPS.