The microbial aromatization of 19-norsteroids for the production of some estrogens was investigated. Thirty different species and strains of bacteria and actinomycetes were screened for their ability to dehydrogenate ring A of 19-nortestosterone with the production of estrone and estradiol. Rhodococcus sp. DSM 92-344 showed the greatest bioconversion efficiency and was selected for further studies. In a medium containing : 3 g/litre beef extract, 5 g/litre peptone, and 10 g/litre glucose, 84% of the substrate was converted after 12 h by 36-h-old Rhodococcus cultures. The maximum total estrogen ouput (77%) was recorded at an initial pH of 7. Presupplementing Rhodococcus cultures with 1 mg/50 mg of substance S (a steroid inducer) before adding the substrate, gave the maximum transformation efficiency after 8 h. The ring-A aromatizing enzymes of the tested bacterium were significantly stimulated in the presence of NADH> ATP> NADPH> EDTA, and significantly inhibited at concentrations ranging from 0.4 to 1 g/litre of either quinone or KMNO(4). All the macroelement chloride salts studied stimulated production of estrone, whereas formation of estradiol appeared to be affected differently. The highest yield of estrone recorded in this work (90%) was attained at 0.8 g/litre CaCl2 concentration. Fe2+ and Mn2+ adversely affected bioconversion. Increasing the concentration of 19-nortestosterone substrate to 50 mg/50 ml reduced the yields of estrone and estradiol; the optimum concentration of substrate, which gave maximum bioconversion efficiency, was 5 mg/50 ml when added in one batch.