The eighth component of human complement (C8) has been purified in high yield from Cohn Fraction III and characterized with regard to its physicochemical properties and subunit structure. The purified product was found to be similar to functional C8 isolated from plasma or serum. Human C8 possesses a molecular weight of 151,000 and is composed of a 1:1:1 ratio of three nonidentical subunits: alpha (Mr = 64,000), beta (Mr = 64,000), and gamma (Mr = 22,000). These subunits occur as a covalently linked alpha-gamma dimer which is noncovalently associated with beta. After purification and characterization of alpha, beta, and gamma, each was found to possess different amino acid compositions and NH2-terminal sequences. Both alpha and beta subunits contain similar but exceptionally high percentages of hydrophobic aromatic amino acids. As measured by circular dichroism, the secondary structure of C8 contains 12% alpha-helix, 24% beta structure, and 64% unordered structure, values typical of globular proteins. Complete secondary structure, as well as hemolytic activity, can be recovered after exposure to 6 M guanidinium hydrochloride or 8 M urea. The alpha-gamma and beta subunits were dissociated and isolated in the presence of sodium dodecyl sulfate and after removal of detergent, neither was found to possess independent hemolytic activity. Significantly, activity equivalent to that of native C8 was generated when alpha-gamma and beta were recombined in an equimolar ratio. These results indicate that C8 is an atypical serum protein with regard to both its subunit structure and denaturation characteristics.