Copper-oxidase activity (ceruloplasmin) was determined, according to Houchin's method, in the blood serum of 65 normal white subjects, aged 20 to 45. The average concentration in this group was 33.4 ± 3.1 mgm./100 ml. In men (n = 40), however, the mean was 32.6 ± 3.1 mgm./100 ml, while in women (n = 25) it was 34.6 ± 2.8 mgm./100 ml; the difference between these means is significant. The dialysed blood serum of 10 normal subjects lost 62.8 ± 0.8 per cent of the copper-oxidase activity. No copper-oxidase activity was found neither in the urine of 20 normal subjects nor in the sperm of other 2 normal subjects.