SUMMARY : Suspensions of the oligotrich protozoan, Metadinium medium, were extracted from whole rumen contents by allowing the large protozoa to settle out from the bulk of the rumen liquor and concentrating by pouring off the super- natant. The settled protozoa were separated from the associated men debris by incubation with maltose, when a bacterial fermentation set in with a Vigorous production of gas which carried most of the rumen debris to the surface for removal by gentle suction. After repeated washing of the protozoa by resuspension in buffer and removal of the supernatant, M. medium was cultivated on powdered cotton-wool (0.05%) and powdered hay (0.05%) in buffer at pH 6.8. These oligotrich protozoa remained alive and dividing for 12 days. A species of Entodinium, concentrated from rumen liquor and washed by centrifugation, after the larger protozoa had settled out, was kept alive for 12-14 days under similar conditions in the presence of rice starch grains instead of powdered cotton-wool. Streptomycin (560 pg./ml.) was used to prepare cultures of protozoa which were bacteriologically sterile when tested by streaking agar plates containing yeast extract, starch and glucose or cellobiose. Under these conditions, Entodinium spp. disappeared within 24 hr. and M. medium in 3 or 4 days. This suggests that these organisms are dependent on bacteria for some of their metabolic processes. It is concluded that the species of Metadinium studied here contained symbiotic cellulolytic bacteria which were destroyed by streptomycin, but that the protozoa continued at least for some time to draw on their polysac- charide reserves after treatment with the antibiotic. Rumen oligotrichs may be of use to the sheep by acting as 'reservoirs' of polysaccharide and so preventing rapid bacterial breakdown. The relationship between the protozoa and the ruminant is considered to be a true symbiosis. There are several records of attempts to cultivate oligotrich protozoa of the rumen contents of ruminants; the most successful attempts were those of Hungate (1942, 1943) in which he kept alive certain Eudiplodinium species for a year and more. Earlier attempts by Knoth (1928), Margolin (1930), and Westphal (1934) met only with very limited success. In the present work, Hungate's technique (1942) was followed quite closely but with different species of oligotrichs. The species were identified from Dogiel's monograph (1927) and Bhatia (1936; taken from Kofoid & MacLennan, 1932). Antibiotics have been widely used as a means of preparing bacteria-free protozoa but with emphasis on the preparation of cultures of flagellate protozoa. The aerobic ciliate, Tetrahymena gekii (now called T. pyriformis), survived treatment with 1000 units streptomycin/ml. (Phelps, 1947); but d'Agostino Barbaro (1951) reported that certain rumen ciliates were killed within 6 hr. by concentrations of streptomycin as low as 400 units/ml. She made no observations on the resistance of individual species. In a previous publication Sugden & Oxford (1952) presented results on the