Balb/c mice were immunized with a human B‐lymphoblastoid cell line typed HLA‐A3, B7. The splenocytes of the immunized mice were fused with a murine myeloma. Supernatants of the cultures were screened against the immunizing cell line in fluorochromasia. Positive cultures were expanded and cloned. One of the clones, X 15.4, was expanded and brought to ascites in Balb/c mice. Monoclonality of the antibody X 15.4, which belongs to the class IgM and immunoprecipites a molecule of 44000 daltons, was demonstrated by isoelectric focusing. By complement dependent citotoxicity the ascites only reacted with the lymphocytes of all HLA‐A3 individuals from a panel of 146 donors, showing no crossreactions. X 15.4 appears to be one of the very rare xenomonoclonal antibodies suitable for HLA t