Abstract: ALTHOUGH tracer studies in a number of carotenogenic systems1,2 have conclusively proved that mevalonic acid is an effective precursor of carotenes, the presence of mevalonic acid has not been demonstrated in any of such systems. In this laboratory an attempt has been made to isolate mevalonic acid from carrots. Previous work3,4 has shown that eell-free extracts of carrots can synthesize carotenes from acetate and mevalonic acid.