1,161,908. Rifamycins. CIBA Ltd. 22 Aug., 1966 [24 Aug., 1965], No. 37442/66. Heading C2A. The invention comprises a 3-substituted derivative of rifamycin S or of rifamycin SV, or of a 16, 17, 18, 19-tetrahydro or 16, 17, 18, 19, 28, 29-hexahydro derivative of rifamycin S or rifamycin SV, the substituent in position 3 being a free or aliphatically substituted amino group, namely, an amino group substituted by one or two monovalent hydrocarbon radicals of aliphatic character, which may be the same or different, or by a divalent hydrocarbon radical of aliphatic character, which hydrocarbons of aliphatic character may be interrupted in the carbon chain by one or more hetero atoms and may be unsubstituted or substituted, e.g. a heterocyclic grouping which is not aromatic; a salt thereof or a quaternary ammonium compound thereof. Examples thereof are 3-morpholino-rifamycin S, 3-pyrrolidino-rifamycin SV, 3-benzylamino rifamycin S, 3-piperidino-rifamycin SV, 3-cyclohexylaminorifamycin S and 3-methylamino-rifamycin SV. Rifamycin S has the following formula The compounds are prepared by reacting rifamycin S, or a 16, 17, 18, 19-tetrahydro or 16, 17, 18, 19, 28, 29 hexa-hydro derivative thereof, with ammonia or with a primary or secondary aliphatically substituted amine, and the 3-substituted derivative of rifamycin S and for the 3-substituted derivative of rifamycin SV so obtained is isolated, a hydroquinone obtained before or after the isolation being oxidized, if desired, to form a quinone and/or a quinone obtained before or after the isolation being reduced, if desired, to form a hydroquinone, and/or, if desired, a resulting compound is converted into a salt. Salts are formed with inorganic and organic acids (listed), and with metals. The picrates may be used in the purification of the bases. A pharmaceutical preparation having antibacterial activity comprises a rifamycin S or SV derivative as above, or a salt or quaternary ammonium compound thereof and a carrier. The compositions may be used orally, parenterally or topically. The rifamycin derivatives may also be added to animal fodders.