Twenty-two borrelidin resistant mutants of Saccharomyces cerivisiae were isolated, studied genetically and their threonyl-tRNA-synthetase was investigated. The borrelidin resistant mutants are classified into four groups. In the first group borrelidin resistance is coupled to the gene HOM3 coding for aspartokinase, in the second group to the gene LEU1. The borrelidin resistance in group three and four is not coupled to anyone of the genetic markers tested. Borrelidin resistance exhibited dominant behavior in all mutants except in the mutant of group 4. The properties of the ThrRS of the mutants of group one, two and four was found to be like the ones of the wild types. However the mutants of group three exhibit a structurally altered ThrRS, which is no longer inhibited by borrelidin.