SummaryA macrophage migration inhibition lest showed that the blood lymphocytes of 15 of 27 sheep inoculatedin uteroor al birth with disrupted cells from ovine malignant lymphomas responded in culture to antigens derived from three other lymphomas. A close association was seen between animals whose lymphocytes were antigen‐sensitive and those which developed lymphocytosis. This association and the specificity of reaction between the various tumour antigens employed in the tests is interpreted as further evidence toward a viral aetiology in ovine malignant lymphoma.