Abstract: A new antibiotic (now called dihydromocimycin) being a yellow, salt-forming weak acid and its non-toxic, pharmaceutically acceptable salts produced by a novel microorganism called Streptomyces ramocissimus, a sample of which is deposited in the collection of "Centraal Bureau voor Schimmel-cultures" at Baarn, The Netherlands, under No. CBS 190.69, or mutants thereof, which antibiotic has antibacterial properties and has been found to be effective against Treponema dysentry, one of the most common swine diseases. Dihydromocimycin may also be converted into mocimycin which is an antibiotic which also possesses growth-promoting properties when added to animal feeds by a dehydrogenation reaction using selenium dioxide as the dehydrogenating agent. The dehydrogenation reaction may be carried out in the presence of a suitable organic solvent.