Abstract The ultracentrifugal behavior and optical properties of porcine ceruloplasmin have been measured to elucidate the conformation of the protein molecule. The molecular weights of apo- and reduced porcine ceruloplasmin are almost identical with that of native protein, i.e., about 150,000, whereas their sedimentation velocities are slightly lower. No dissociation of the protein molecule into subunits occurs upon removal or reduction of the copper atoms, while minor unfolding seems to occur in the apoprotein and further extended unfolding occurs in the reduced protein molecule. Measurements of CD and ORD 4 revealed that native ceruloplasmin shows a small positive CD band at 590 mμ and a negative one at 450 mμ, whereas these bands disappear completely upon removal or reduction of the copper atoms. The ultraviolet CD and ORD data indicate the presence of β-structure in the protein moiety of native, apo- and reduced ceruloplasmin; a distinct negative band at 219 mμ and a positive one at 199 mμ are observed in the CD spectra, while the Cotton effect shows a trough at 230 mμ and a peak at 208 mμ, with crossover at 220 mμ. From these optical assays it is estimated that ceruloplasmin is a composite of β-structures and random coil. This is also evidenced by infrared spectroscopic analysis.