Incorporation of [3H]mevalonic acid-derived materials into proteins was studied with extremely halophilic archaebacteria, Halobacterium halobium and Halobacterium cutirubrum. Several labeled proteins were detected on SDS-polyacrylamide gel electrophoresis followed by fluorography. The majority of the radioactive materials released from the labeled proteins by sulfonium salt cleavage moved with a mobility similar to that of a C85 polyprenol on reverse-phase thin-layer chromatography, and no radioactive farnesol was found on the chromatography. However, a weak but significant protein farnesyltransferase activity was detected in in vitro experiments with a combination of [3H]farnesyl diphosphate and Ras precursor protein.