This paper examines the expression of CD1 in the sheep utilising the monoclonal antibodies (mAbs) which were assigned to OvCD1 in the First and Second Workshops on Ruminant Leukocyte Differentiation Antigens along with those primarily clustered as Bov/OvCD1 in the Third Workshop. Detailed immunohistological studies of both lymphoid and non-lymphoid tissues and flow cytometry of isolated cell populations revealed two distinct patterns of CD1 expression in the sheep. The mAbs assigned to the sub-cluster BovCD1w1 (SBU-T6) and BovCD1w3 (IAH-CC43 and IAH-CC118) were much more widely distributed than those of the sub-cluster BovCD1w2. In addition to cortical thymocytes and dendritic cells (DC) the CD1w1 and w3 molecules are expressed by peripheral blood B lymphocytes, monocytes and many tissue macrophages.