This paper describes three anti-sheep monoclonal antibodies. The tissue distribution and apparent molecular weight of the antigens detected by these antibodies is consistent with them reacting with sheep leucocyte common antigen (CD45 (VPM18], MHC class I (VPM19) and CD5 (VPM29). An ELISA method is described that permits the cross-reactivity of different antibodies to be assessed, this confirms the identity of the antigens detected by VPM18, VPM19 and VPM29. This method is also of value as either a positive or a negative screen in the construction of further monoclonals. A study of the expression of these three antigens on efferent lymph small lymphocytes and antigen-activated lymphoblasts shows that the density of CD45 on lymphoblasts (activated either in vivo or in vitro) is approximately half that of small lymphocytes whereas the density of MHC class I is the same in both populations. Furthermore, about 75% of small lymphocytes express CD5 but less than 10% of lymphoblasts are positive. Cell membrane CD5 expression is lost on lymphocyte activation. It does not seem to be linked to cell membranes via phosphatidylinositol and the loss is not due to the breaking of that link.