Forty-six percent of 149 reagents against class 1 antigens of the bovine major histocompatibility system reacted with lymphocytes from 1 or more sheep in a microlymphocytotoxicity test. The reactions fell into six groups. The majority of antisera defined an antigen present on both erythrocytes and lymphocytes. This antigen is probably the R blood group antigen and the reactions were unrelated to the presence of anticlass I activity and are probably due to the additional presence of antibodies of R specificity. The other antigens are not identical to any of the 14 red blood cell factors or seven lymphocyte antigens examined, nor did they correspond to any of the class I bovine lymphocyte antigens defined by the alloantisera.