Certain temperature-sensitive (ts(-)) mutants of yeast which cannot be corrected by nutritional supplementation exhibited a rapid cessation of protein synthesis after a shift to the restrictive temperature. Genetic and biochemical tests permitted a division of these mutants into four classes. This division was based upon genetic complementation patterns among the mutants and an investigation of glucose incorporation into macromolecules and polyribosome content in the mutants after a shift to the restrictive temperature. A study of these parameters in the parent strain (ts(+)) in the presence of certain well-characterized inhibitors allowed a tentative identification of the biochemical defects in each of the four classes. The properties of the mutants in class IA were consistent with the hypothesis that they result from a defect in the initiation of polypeptide chains or in ribonucleic acid synthesis; mutants in class IB from a defect in the elongation of polypeptide chains; mutants in class IIA from a defect in energy metabolism; and mutants in class IIB from a lesion affecting membrane function.