Enzyme-linked immunosorbent assay (ELISA) methods for measuring murine serum amyloid A (SAA), a representative acute phase reactant, were developed utilizing a newly produced monoclonal antibody. Two site-ELISA, in which the monoclonal antibody was used as the captured antibody, was sensitive enough to determine the SAA concentration in mice at the steady state. Direct binding ELISA, in which the sample SAA bound to the plastic wells was detected by the antibody, was simple and suitable for measuring the elevated SAA, but could not analyze the resting level of SAA because of the need for high dilution in plasma samples. Plasma SAA concentrations were measured in ten ICR mice on the day of purchase and at the end of seven days of ordinary rearing. the SAA concentration of one animal decreased from 1.6 to 0.5 mg/1 during a week, while the others had no obvious changes. the plasma SAA of the ten animals after one week of rearing ranged from 0.3 to 0.8 mg/1 with a mean of 0.47. These mice, two days after 10 μ g lipopolysaccharide were given, had increased SAA values up to a mean of 300 mg/1, though with variations between animals.