Abstract A monoclonal murine anti-HLA-B27 antibody (designated anti-B27M 1 ) has been produced by the technique of somatic cell hybridisation. When tested by standard lymphocytotoxicity against panels of normal controls and patients with spondyloarthropathies, anti-B27M 1 showed as good a correlation with the HLA-B27 alloantigen as did conventional typing alloantisera (r= 0·9). No differences were detected between the typings of patients and of controls, suggesting that the B27 alloantigens of each group share a common epitope and that this specificity may not be divisible into normal and disease-associated variants. These results show that specified hybridoma antibodies of reagent quality can be produced and applied to HLA typing