The present invention relates to a process of separation of 4-aza-androst-1-ene-17-oic acid from 4-aza-androstan-17-oic acid, which brings to 4-aza-androst-1-ene-17-oic containing less than 0.05% w/w of 4-aza-androstan-17-oic acid, with high yield and productivity. In particular, the present invention relates to a process for the separation of 4-aza-androstan-17-oic acid from 4-aza-androst-1-ene-17-oic acid, comprising the steps of treating the crude 4-aza-androst-1-ene-17-oic acid with formic acid and recovering the purified 4-aza-androst-1-ene-17-oic acid containing 4-aza-androstan-17-oic acid in w/w % less than 0.05%.
