The human cytotoxic hybridoma antibody 4166 (IgM kappa) was generated by fusing an in vitro EBV-transformed B-LCL from a multiparous woman with the mouse-human heteromyeloma fusion partner CB-F7. In microcytotoxicity and IIF tests with B-LCLs as target cells, the mAb 4166 was specific for DQ3 (= DQ7 + 8 + 9). However, when used for DQ typing of class-II-positive PBMCs, 4166 could be rendered functionally specific for DQ7 + 8 and did not react with DQ9+ PBMCs. Binding of mAb 4166 to DQ8-positive cells was efficiently blocked by several allotype-specific mAbs recognizing DQ8. Other HLA class-II-specific mAbs were unable to inhibit. With the use of mAb 4166, it is possible to discriminate DQ7 + 8 from DQ9 in serologic DQ typing.