The anti-CD19 antibody-producing hybridoma clone BU-12 was produced by the immunisation of two BALB/c mice with the Burkitts lymphoma cell line EB4. Immune spleen cells from these animals were fused with the mouse X63 AG8 653 plasmacytoma cell line and resulting hybridomas selected in HAT medium. Screening of hybridoma culture supernatants on a range of T-, B-, myeloid and erythroid cell lines and on sections of normal human tonsil led to the selection of the pan-B-cell-reactive BU-12 hybridoma clone.