SUMMARYThe reactivities of peroxidase-labelled concanavalin A (con A: 20 jug/ ml) with sections from various avian lymphoid organs were compared. With chicken bursae, con A binding and subsequent differential staining detected a non-lymphoid leukocyte population distributed throughout the sections. A comparable population was not observed in sections from the gland of Harder. With free cells, con A at low concentrations was found to selectively bind to both macrophages and heterophils but not to lymphocytes. Flow cytometric analysis with fluorescein isothiocyanate-labelled con A . also confirmed this selective binding.Peroxidase-labelled con A was used to detect non-lymphoid leukocytes present in areas of metastasised tumours in tissues of quail-chicken hybrids. Such staining was not seen in comparable normal tissues. Evidence that the tumour-localised leukocytes were macrophages was shown by the reactivity of these cells with a monoclonal anti-chicken la antibody.