The diagnostic value of a panel of monoclonal antibodies was assessed in 100 consecutive patients with acute leukaemia. 97 patients were clearly phenotyped. Clinical and haematological feedback showed that the immunological data made a critical contribution to the final haematological diagnosis in 19 patients and provided useful confirmatory data in another 78. Immunophenotype also provided the basis for a subset classification of known prognostic relevance to acute lymphoblastic leukaemia (ALL). Immunophenotype and haematological findings conflicted in 2 cases, and 3 cases were unclassifiable with the antibody panel. In these difficult cases leukaemic-cell DNA was investigated for immunoglobulin gene rearrangement. Immunophenotyping with selective probes may be used in conjunction with other laboratory analyses (eg, karyotyping) in the routine investigation of patients with acute leukaemia.