A murine allo-immune A.TH anti-A.TL (anti-Iak) serum pool and sera from individual immunized mice were tested on a large panel of human DR typed B cells, obtained from peripheral blood (PBL-B cells), and human lymphoid tumour cells. The anti-Iak serum pool appeared to be a useful reagent both in cytotoxicity and immunofluorescence for the examination of the presence of class-2 HLA molecules on tumour cells. Only one discrepancy was observed when the pool was tested in parallel with a xeno-anti-human B-cell serum on the cells of 56 lymphoid tumours. Nine sera from individual mice were tested on a panel of 93 PBL-B cells. Some of these sera reacted significantly more weakly with human B cells positive for the DR5 and/or DR7 antigens.