Eight cases of 'hairy' cell leukaemia (leukaemic reticuloendotheliosis) were evaluated by scanning electron microscopy. The surface of critical-point-dried 'hairy' cells was characterized by prominent and exaggerated broad-based, ruffled membranes and scattered small clusters of stub-like microvilli. The surface morphology resembled that of normal and leukaemic monocytes, but differed from that of normal and leukaemic lymphocytes. Some cells with features of both lymphocytes and monocytes were difficult to categorize; the overall impression of the surface architecture of most 'hairy' cells suggests, however, that they are related to the monocytic series. From the examination of these cases it is evident that scanning electron microscopy may be used as a means of distinguishing chronic lymphocytic leukaemia from 'hairy' cell leukaemia on the basis of surface ultrastructure.