Paraffin sections of a variety of tissues from 12 patients with typical hairy-cell leukaemia (HCL) were stained for immunoglobulin heavy and light chains by the peroxidase-antiperoxidase (PAP) technique. Plasma cells were frequent, particularly in a lymph node from a severely infected patient. The reactive nature of the plasma cells of HCL was suggested by the fact that there was no restriction of light-chain expression, although viable hairy cells were shown to express monoclonal surface immunoglobulin. This, together with the absence by both light and electron microscopy of forms intermediate between hairy cells and plasma cells and the lack of ribosome-lamella complexes in the plasma cells, suggested that hairy cells do not differentiate into plasma cells. Although hairy cells are known to contain immunoglobulin, this was not demonstrable in hairy cells in the paraffin-embedded tissue. The PAP technique was also useful for demonstrating abundant splenic macrophages in HCL.