1. Human serum apolipoprotein A-I contains a prominent 11-residue sequence periodicity. 2. Similar 11-residue segments occur in the other sequenced human apolipoproteins, C-I, C-III, and A-II. 3. Computer analyses of the sequences support the hypothesis that they evolved from a common ancestor. 4. An evolutionary history of these proteins is proposed. 5. The estimated rate of change of these proteins indicates that all four types will be found throughout the vertebrates and that related proteins will also be found in invertebrates.