An isomerization method wherein a compound of formula ##STR1## wherein X and y together form a 3-oxo radical, or X is a hydrogen atom and Y is either a radical a ##STR2## where R.sub.2 is OH or O-acyl, or an O-alkyl or NH.sub.2 radical; R.sub.1 is a hydrogen atom or a methyl radical; Z is hydrogen or an acyl radical; and R is a hydrogen atom, an NH.sub.2 radical or a (CH.sub.2).sub.n Ar, NH(CH.sub.2).sub.n Ar or N.dbd.CH(CH.sub.2).sub.n Ar radical, in the form of a 10.alpha. isomer or a mixture of 10.alpha. and 10.beta. isomers, is exposed to a basic agent to give the corresponding compound of formula I in which the 10-methyl radical is in the .beta. position.