Auxin, the growth substance which promotes growth by increase in cell size, and which has been obtained in crystalline form from human urine, has the formula C18H32O5, and is a monobasic acid. The following derivatives were prepared: methyl and p-phenylphenacyl esters, tri (m-dinitrobenzoyl) auxin, dihydroauxin, and dihydro-auxinlactone. Auxin becomes inactive as a growth promoter after several months even if kept under vacuum in the dark. Preliminary studies indicate that inactiva-tion results from molecular rearrangement.