A survey of a number of animals showed that the formate activating enzyme is present in almost all tissues. In invertebrates, the largest amounts of this enzyme were found in the gonads and reproductive products and the smallest amounts were found in muscle and digestive glands, except for Cancer magister. In the few vertebrate species included in this study, the greatest amounts of formate activating enzyme were found in liver tissues. Formate activating enzymes isolated from the oöcytes of Schizobranchia insignis, from the ovaries of Pecten caurinus, Balanus nubilis and Cancer magister and from the livers of Sebastodes caurinus and the rabbit were compared with respect to the KM values for the three components of the reaction and for the requirement for divalent metal ions. Some information was also obtained on the pH optimum, the require-metal ions. Some information was also obtained on the pH optimum, the requirement for a reducing agent, and the stability of various enzyme preparations to storage in the cold. The data obtained indicate that the above enzymes are similar to highly purified preparations of formate activating enzyme obtained from bacteria, although some differences were found in the individual KM values, the metal requirements and the stability upon storage. Forty-one tissues from a selected group of animals (Schizobranchia insignis, Nereis brandti, Pecten caurinus, Strongylocentrotus purpuratus, Balanus nubilis, Cancer magister, Sebastodes caurinus, Onchorynchus nerka, the chicken and the rabbit) were examined with respect to the amounts of four additional enzymes (N10-formyl tetrahydrofolic acid deacylase, cyclohydrolase, hydroxymethyl tetrahydrofolic acid dehydrogenase, and serine hydroxymethylase) concerned with the synthesis and interconversion of one-carbon derivatives of folic acid. Except for muscle tissue, varying amounts of the four enzymes were found and no correlation could be found between the levels of these enzymes and that of the formate activating enzyme. The muscle tissues of the above animals contained only small quantities of formate activating enzyme, deacylase, cylohydrolase and hydrofolic acid dehydrogenase, but had fairly large amounts of serine hydroxymethylase. In addition, the above animals were tested for the presence of the formaldehyde activating enzyme. Although 29 of the 41 tissues were found to have this enzyme, no correlation could be found between its distribution and that of the other enzymes studied.