Different mutants of the catabolism of androst-4-ene-3,17-dione were produced in Nocardia restricta by action of y-rays and nitrosoguanidine. These selected mutants (acetate+-undrost-4-ene-3,17-dione-) are hexanoate-. Their behaviour is identical to the mutants selected as hexanoateacetatet which are themselves incapable of growing on androst-4-ene-3,17-dione. These mutanta degrade androst-4-ene-3,17 dione to methylperhydroindane dione propionic acid in presence of acetate with a molar yield of about 600%. The study of the biosynthesis of the 1,f-steroid-dehydrogenase in the mutant strains shows that a metabolite of MEPHIP is an inducer of the biosynthesis of this enzyme. On the other hand, the behaviour of these mutant strains shows an inductible poxidation mechanism which takes place in the catabolism of methyl perhydroindane dione propionic acid. Another isolated mutant strain degrades androst-4-ene-3,17-dione with an transient accumulation of 3-hydroxy-9,10-secoandrosta-1,3,5(lO)-t~ene-97l7-~one.