Rat ceruloplasmin was purified by a three-step column chromatography procedure, utilizing DEAE-Sepharose, Sepharose CL-6B, and CM-Sephadex A50 columns. The molecular weight of rat ceruloplasmin determined by a molecular sieve column was 124,000 daltons. An optical density ratio (610 nm/280 nm) of 0.051 and a molar extinction coefficient of 8600 were obtained. A decrease in lysine in rat ceruloplasmin compared with human ceruloplasmin could account for its reduced anodal mobility. Other differences in the amino acid sequence of the rat ceruloplasmin included an increase in methionine and cystine/cysteine, and a decrease in histidine, tyrosine and tryptophan.