The hybridoma technique was used to produce a monoclonal antibody specific for a polymorphic determinant on human la antigens. BALB/c mice were immunized with a B-lymphoblastoid cell line derived from an HLA-DR3/DR4 positive donor. Hybridoma culture supernates were screened in two stages by means of the microcytotoxicity assay. Supernates were first tested against the la-positive immunizing cell line and against la-negative cells. Cultures identified as producing antibody cytoxic for the immunizing cell line but not for la-negative cells were subcultured and the supernates then tested against a selected panel of HLA-D homozygous cell lines (HCL). In this manner, it was possible to identify a microwell of hybrid cells producing an antibody that reacted with a polymorphic HLA-DR determinant. Cells from this culture, designated 17.15, were cloned twice by limiting dilution. Specificity was evaluated with a panel of 39 HCL and B cells from 70 normal donors. Antibody 17.15 recognized a supertypic DR "4 + 5" specificity, which was also present on DRw9 and some DR7 positive cells. These findings suggest that the cross-reactivity groups DR "4 + 5" (MB-3) and DR "4 + 7" (MT-3) defined by alloantisera may share a common supertypic specificity that is recognized by monoclonal antibody 17.15.