Cells from 17 patients with acute myeloid leukaemia (AML) were investigated using the following IgG2a monoclonal antibodies cALL, B1 and Leu 1. Cells from six patients showed binding of cALL, five showed B1 and three showed Leu 1 positivity. Binding of the monoclonal antibodies was shown to be due to Fc binding as it was blocked by prior incubation with aggregated human IgG. Measures such as prior incubation with AB serum or incubation at 37 degrees C were insufficient to block Fc binding in all cases. Fc binding correlated with evidence of monocytic differentiation. Our findings suggest that unless Fc binding is outruled false positive results may be obtained in the investigation of lineage infidelity in AML particularly in monocytic leukaemias.