The invention relates to a novel metabolite, M.139603, of the empirical formula C.sub.35 H.sub.53 O.sub.8 Na, which is obtained from the aerobic fermentation of a novel Streptomyces species, NCIB 11426, and to the corresponding free acid form thereof, C.sub.35 H.sub.54 O.sub.8. M.139,603 reduces the proportion of methane produced by the "digestion" of food in the rumen of ruminant animals and increases the proportion of propionate to acetate in the rumen contents. The compound is therefore useful for increasing the growth rate of ruminant animals, and/or for increasing their efficiency of food conversion. The compound also possesses activity against Gram positive micro-organisms, and against coccidia, for example Eimeria tenella.