The propagation of a cell line from a rabbit affected with the sheep associated form of malignant catarrhal fever is described. Immunological and morphological characteristics of the cell indicated that it was a T-lymphocyte and the presence of electron dense cytoplasmic granules suggested that the cell could be further classified as a large granular lymphocyte. The cell line required a feeder layer and was cytotoxic to both primary cell cultures and cell lines, a characteristic of large granular lymphocytes. No evidence of the nature of the agent could be detected but as few as 10(2) cells transmitted the disease. These findings are discussed and the possibility that infection and subsequent dysfunction of large granular lymphocytes may have a central role in the pathogenesis of malignant catarrhal fever is considered. That cells with similar characteristics have been derived from Herpesvirus saimiri and H ateles infected marmoset lymphocytes suggests that the lymphoproliferation associated with infection by these two simian herpes-viruses and malignant catarrhal fever may have a similar pathogenesis.