The modification of Middlebrook's 7H11 oleic acid albumin agar medium by the addition of fresh bovine serum and lysed sheep red cells to encourage growth of Mycobacterium bovis is described. The improved medium was made selective by the addition of antibiotics and a comparative trial of this medium and the guineapig test in the isolation of M. bovis from badger tissues is reported. A close agreement between the two tests was found; the guinea-pig test detected 95% of all isolations and culture detected 91%.