Data from batch growth curves of mouse LS cells cultivated at controlled dissolved oxygen partial pressures were used to calculate the weight of cells produced per mole of adenosine triphosphate generated (YArv). These values agree well with those reported for bacteria. A theoretical relationship was developed which allowed the biosynthetic and maintenance energy requirements to be estimated. The biosynthesis of LS cells required i-6 x io" 1 moles of ATP/cell. The maintenance energy, which is a function of growth rate, was 2-9 x io"" moles ATP/new cell when the mean generation time was 1-15 days. The proportion of the total energy used for maintenance under these conditions was 65 %. This corresponds to a value of less than 10% for bacterial maintenance when the organisms are grown at near their maximum rate. A comparison of biosynthetic energy requirements indicates that bacteria and moulds require about 4 times as much energy as animal cells to generate the same weight of cell material. Possible explanations of this difference are discussed.