UBIQUINONE, a naturally occurring quinone of considerable biological interest, was first isolated by Morton et al. 1, who found it in many kinds of animal tissue. Lester et al. 2 have shown that a series of such quinones exist, and have isolated, from different organisms, five compounds of general formula : differing from each other by single isoprene units (n = 10, 9, 8, 7 and 6). Further studies3 suggested that ubiquinone 50 (n = 10) was the normal quinone in higher mammals, but Ruegg et al. 4 have recently shown that rat liver contains ubiquinone 45 (n = 9).