We developed an ELISA for human serum MASP-1, a Cls-like serine protease which is known to function in C4 and C2 activation. We then determined MASP-1 levels in 1063 sera from normal Japanese subjects ranging in age from 3 to 100 years, as well as in certain body fluids using this assay. Individual serum MASP-1 levels ranged from 1·48 to 121·83 μg/ml, with a normal frequency distribution pattern. The arithmetic mean ± s.d. of MASP-1 levels in serum was 6·27 ± 1·85 μg/ml, whereas levels of MASP-1 in cerebrospinal fluid and in urine were almost undetectable. When the mean ± s.d. of serum MASP-1 was calculated for each age group (10 year range) and values were then compared, the age group consisting of 3–9-year-olds (7·54 ± 1·39; μ/ml) was found to have the highest value. When MASP-1 was measured in cord blood, it was shown that levels were already as high as those of 3–9-year-olds. The serum MASP-1 level was found to be as strongly dependent on age as is the serum MBL level. MASP-1 and MBL are thought to play an active part in immunity in younger people. It was found that the serum level of MASP-1 was much higher than that of MBL, and the major portion of human serum MASP-1 appeared to exist in the circulation as a form unbound to MBL.