A procedure based on modifications of published methods for human proteins for the isolation of rat C8 and C9 from one batch of serum is described. The procedure allows the rapid, large-scale isolation of pure and haemolytically active proteins. Rat C9 had a slightly higher molecular weight than human C9 on SDS-PAGE and similar isoelectric point. Rat C8 differed from human C8 in the molecular weight of the y chain (23,000 and 21,000 kD respectively), and on isoelectric focusing (pi rat C8: 6.5-6.9; pi human C8: 7.4-7.9).