Microcytotoxicity tests with the sera from renal transplant recipients against cultured B-cell lines were performed by using guinea pig complement. Out of 106 sera, 18 sera showed strong cytotoxicity against cultured cell lines. The same results were obtained by using normal human serum as a complement source. These sera did not show any cytotoxicity against normal peripheral T- and B-lymphocytes. On the other hand, the sera containing the cytotoxicity against cultured B-cell lines were found to have high-titered heterophile antibodies against bovine red blood cells (BRBC) when hemolysis in agar gel with guinea pig complement was used. The sera without cytotoxicity did not have high-titered heterophile antibodies. The sera with cytotoxicity against cultured B-cell lines reacted in the same way against both human tonsilar lymphocytes cultured with the medium containing fetal calf serum and human serum. Therefore, the antigenicity of cultured cells was not affected by the fetal calf serum in the culture medium. After absorption with BRBC, the sera lost their cytotoxicity against cultured cells. Hemolysins in the sera were almost completely removed after absorption of the sera with cultured cells. From these results, it seemed apparent that the sera from renal transplant recipients contained heterophile antibodies which reacted with the cells of some cultured B-cell lines and tonsilar lymphocytes.