A monoclonal antibody, FMC 5, has been prepared against human lymphocytes and its reactivity examined. FMC 5 behaves, for the most part, as an anti-HLA-A2 typing serum. However, three small groups of discrepant types were identified, and each provides additional insight into the complexity of the MHC antigens. A small number (5/60) of donors typed as HLA-A2 did not react with FMC 5. A further group (6/35) were HLA-A2 and did react with FMC 5, but lysis was always incomplete in this group. Finally, 13/39 non-A2 donors gave various degrees of reactivity with FMC 5. Lysis in this group was usually incomplete and required a high antibody concentration.