This paper describes a monoclonal antibody which reacts with transfected cells carrying a gene (NCA-50/90) which has been shown to encode the human CD66 antigen. However, antibody By114 recognizes only a single 90 kD polypeptide from human neutrophils, whereas the antibodies which originally defined the CD66 antigen also recognize a larger 180-200 kD protein. We conclude that antibody By114 is selective for the smaller of the two CD66 gene products, which is a surface membrane phosphatidyl-inositol (PI)-linked molecule. The reactivity of antibody By114 on peripheral blood cells (positive on neutrophils, weak or negative on eosinophils, and negative on basophils, monocytes and lymphocytes) and myeloid precursor cells was identical to those of a reference CD66 antibody, as was the staining of leukaemic cells. However, the reactions of the two antibodies differed on kidney, liver and pancreas, and in cases of myeloma, Waldenström's macroglobulinaemia and lymphoma, indicating that By114 represents a new CD66 sub-specificity. Granulocytes from a case of paroxysmal nocturnal haemoglobinuria (PNH) were negative with antibody By114, indicating that it may be of value in detecting the defect in PI-linked surface molecules characteristic of this condition. Antibody By114 also stained formalin fixed paraffin embedded tissues and may therefore be of use in routine diagnostic histopathology.