Our original objective in studying lamprey fibrinogen was embodied in the notion that the proteins of this ancient vertebrate might themselves by more primitive. As such, it was possible that the subunits of lamprey fibrinogen might have been more similar, one to another, than is the case in higher vertebrates, or even identical. Amino acid analysis of the individual polypeptide chains indicates, however, that the alpha, beta and gamma-chains are instead more dissimilar from each other than are the corresponding chains from human fibrinogen. This finding was somewhat surprising because regions of homology have been detected recently among those three chains when isolated from human fibrinogen, suggesting that all three chains have indeed descended from a common ancestor. The paradox is especially evidenced by the unusual amino acid composition of the lamprey alpha-chain, 45% of which is composed of glycine, serine and threonine. This unusual amino acid distribution may be involved in the anomalous behavior of these chains on sodium dodecyl sulfate polyacrylamide gel electrophoresis.