A .DELTA..sup.1,4,17 -BNC compound having the formula: ##STR1## wherein X is a substituent selected from the group consisting of OH and OCH.sub.3 ; as well as its process of production by microbiological side-chain splitting of a 17-C-side-chain steroid with a defect mutant microorganism which gives a steroid with a 17-C-.alpha.-propionic acid substituent in the absence of inhibitors inhibiting either the cleavage of the steroid ring or microorganism growth, where the defect mutant microorganisms are grown under a reduced air supply whereby an enrichment of .DELTA..sup.1,4,17 -BNC occurs.