New antibiotically active compounds having the basic structure of rifamycin S, namely 3-hydroxyrifamycin S (formula A: X=>C.dbd.O; R.sup.1 =OH; R.sup.2 =H), 3,31-dihydroxy-rifamycin S (formula A: X=>C.dbd.O; R.sup.1 =R.sup.2 =OH) and 1-desoxy-1-oxarifamycin S (formula A: X=--O--; R.sup.1 =R.sup.2 =H) ##STR1## are formed by cultivating, under aerobic conditions, a strain of Nocardia mediterranei which is derived from Streptomyces mediterranei ATCC 13 685 as the parent strain and is characterized by the ability to produce at least one of the mentioned compounds. The recombinant strain Nocardia mediterranei DSM 1415 has proved suitable. The mentioned rifamycin S analogues have analogous antibiotic properties to this but have a wider range of action, especially against gram-negative bacteria.