Colostrum-free newborn pigs were raised artificially to investigate the appearance of ceruloplasmin in the serum of piglets. The sensitivity of the p-phenylenediamine oxidase activity assay, commonly used as a measure of ceruloplasmin, has been increased 10-fold. Based on this activity, no evidence was found that this copper protein was present in the serum of baby pigs at birth, although some evidence was obtained that the protein moiety (apoceruloplasmin) was present. Holoceruloplasmin usually became detectable in the serum 10-15 hours after birth. Its concentration increased slowly at first and then at an accelerated rate. The ceruloplasmin in the sera of 2-3-day-old piglets was isolated and compared with that isolated from the serum of adults. Contrary to a previous report, the physical and chemical properties of ceruloplasmin synthesized during the first 3 days of life of the piglet showed no significant difference from that synthesized by the adult.