The bacterial superantigen staphylococcal enterotoxin A (SEA) is a highly potent activator of cytotoxic T cells when presented on MHC class II molecules of target cells. Our earlier studies showed that such SEA-directed T cells efficiently killed chronic B lymphocytic leukemia (B-CLL) cells. With the ultimate goal to replace the natural specificity of SEA for MHC class II molecules with the specificity of a monoclonal antibody (mAb), we initially made a mutated protein A-SEA (PA-SEAm) fusion protein with > 100-fold reduced binding affinity for MHC class II compared to native SEA. The fusion protein was successfully used to direct T cells to B-CLL cells coated with different B lineage specific (CD19, CD20) or associated (CD37, CD40) mAbs. The PA-SEAm protein was 10-100-fold more potent against mAb coated compared to uncoated HLA class II+ B-CLL cells. No correlation was seen between the amount of mAb bound to the cell surface and sensitivity to lysis. Preactivation of B-CLL cells by phorbol ester increased their sensitivity, and lysis was dependent on ICAM-1 molecules. However, no preactivation of the target cells was needed when a cocktail of two or four mAbs was used. Circulating leukemia and spleen cells were equally well killed. We conclude that the natural target specificity of SEA, MHC class II, can be reduced by mutagenesis and novel binding specificity can be introduced by linkage to tumor reactive mAbs. Our findings encourage the construction of recombinant SEA mutant fusion proteins for specific T cell therapy of hematopoietic tumors such as B-CLL.