For the production of 17-C-steroid- alpha -propionic acid compounds by microbial side chain degradation on 17-C-side chain steroid substrates wild strain microorganisms selected in predetermined manner are subjected to a multi-stage mutation and selection. The resultant mutant strains are capable of supplying the desired compounds, e.g. DELTA -4 BNC and/or DELTA -1,4 BNC in high yields even in the absence of inhibitors arresting the growth and/or degradation of the steroid ring. Wild strains and mutants particularly suitable, and their choice, production and use, are described.
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