The study of large groups of individuals (from reactive arthritis epidemics, for example) for epidemiologic and genetic data has been hampered by the techniques by which HLA typing is performed. Microlymphocytotoxicity is costly not only in materials but also in the time required to perform a given assay and thus does not lend itself to large scale testing. To study such epidemics, in the best way, one must be able to respond as soon as possible after the epidemic is recognized and on a large scale. We thus examined HLA-B27 typing by flow cytometry and commercially available monoclonal antibodies. Two commercially available monoclonal HLA-B27 antibodies were compared. We conclude that the utilization of monospecific HLA-B27 antibodies and flow cytometry allows for the determination of HLA-B27 and HLA-B7 phenotype of a large number of individuals over a short period of time, thereby making the study of a large group both economical and feasible.