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 output (77%) was recorded at an initial pH of 7. Presupplementing Rhodocossus 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 CaCl 2 concentration. Fe 2 + and Mn 2 + 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.