The transfer of sulfur from methionine to cysteine in the archaebacteria Sulfolobus acidocaldarius and Halobacterium marismortui was studied by feeding S-34-labeled methionine to cells and measuring the incorporation of S-34 into protein-bound cellular cysteine and methionine by mass spectrometry. It was found that, as are eucaryotes, both of these archaebacteria were able to convert the sulfur of methionine to cysteine.