The initiating complement component (C1q) in the classical pathway of 730 subjects and the first essential component (C3) in the alternative pathway of 461 subjects in Japan were examined. The study population consisted of normal healthy newborns, infants, children, adults and the old (from birth up to 75 years of age). In cord sera, both C1q and C3 were about 60% to the total mean level. At 3 days of age, C1q markedly increased to the mean level which remained relatively invariable up to 40 years of age. And above 40 years, the C1q level increased steadily with age up to about 75 years of age. C3 reached the mean level at about 1 month of age, and was highest during infancy. This level declined at about puberty and then continued to increase steadily with age up to around 55 years. In normal healthy subjects, a moderate positive rank correlation was found between C1q and C3 amounts. No significant differences of C1q and C3 levels were evident between male and female. No C1q was demonstrable in the colostrum from which lipids were previously removed, but after concentration by precipitation in a chelating agent with a low ionic strength, C1q, measured immunochemically, was detectable at concentrations of 300 ng/ml of colostrum. C3 was also detected at concentrations of about 200 microgram/ml of colostrum.