Synthetic phylloquinone was resolved into cis and trans isomers by thin-layer chromatography. The two isomers had identical ultraviolet spectra characteristic of vitamin K1 and were differentiated by nuclear-magnetic-resonance spectroscopy on the basis of the displacement of the peak corresponding to the olefinic methyl group in the naphthoquinone side chain. Studies on the restoration of electron transport coupled to phosphorylation in irradiated preparations of Mycobacterium phlei showed that only the trans isomer was active with substrates linked to nicotinamide-adenine dinucleotide. The purified trans phylloquinone was enzymatically converted to the cis isomer. Under similar conditions, cis vitamin K1 gave rise to the trans-naphthoquinone. The natural naphthoquinone of M. phlei vitamin MK9(II-H) was similarly resolved into cis and trans isomers.