Expression of surface membrane receptor for IL-6. The expression of the receptor is assessed by the binding of PE-labeled human-recombinant IL-6 (FL2). MKs are identified by simultaneous labeling with LJ-P4 MoAb directed to the membrane CD41 (GPIIb/IIIa complex). MKs (marked with a box) exhibit an enhanced double fluorescence compared with other marrow cells.