Gorman, G. C. (Mm. Vert. Zool, Univ. Calif., Berkeley, 94720), A. C. Wilson (Dept. Biochem., Univ. Calif., Berkeley, 94720), M. Nakanishi (Dept. Biochem., Univ. Calif., Berkeley, 94720) 1971. A biochemical approach towards the study of reptilian phytogeny: Evolution of serum albumin and lactic dehydrogenase. Syst. Zool., 20:167–186.—Albumin was purified from blood plasma and the FL isoenzyme of lactic dehydrogenase was purified from heart tissue of the lizard Iguana iguana. The purified proteins behaved as single components upon gel electrophoresis. They were injected into rabbits. The resulting antisera were tested for reactivity with unpurified preparations of albumin and heart lactic dehydrogenase from more than 50 other species. Reactivity was detected qualitatively by immunodiffusion tests and quantitatively by micro-complement fixation tests. Antisera against iguana Ht lactic dehydrogenase reacted strongly with the heart lactic dehydrogenases of other lizards as well as with those of snakes. The heart lactic dehydrogenases of the “living fossil” Sphenodon, crocodilians, turtles and birds reacted weakly. No reaction was detected with amphibian or mammalian heart lactic dehydrogenases. Antisera against iguana albumin reacted strongly with those of iguanine species, less strongly with those of other iguanid species and weakly, if at all, with those of non-iguanid lizards and snakes. Thus, by immunological criteria, albumin evolves faster than the H4 enzyme. Quantitative studies suggest that the immunological properties of albumin have changed about twice as fast as those of LDH during lizard evolution. Furthermore, lactic dehydrogenase appears to have evolved in a less regular fashion than has albumin. The quantitative immunological comparisons also demonstrate that as much albumin evolution has gone on in the family Iguanidae as in the mammalian orders Primates or Carnivora. Since lizard families, according to the fossil record, are approximately as old as orders of placental mammals it appears that albumin evolution has been proceeding at about the same rate in iguanids as in primates and carnivores. Similar studies were conducted with rabbit antisera prepared against purified alligator albumin. The results of cross-reactivity studies with the albumins of other crocodilians fit with the suggestion that rate of albumin evolution in crocodilians has been similar to that in iguanids, primates and carnivores.