We have used monoclonal antibodies AP-1 (anti-GP Ib alpha). AP-2 (anti-GP IIb-IIIa) and FMC 25 (anti-GP IX) in immunofluorescence and immunocytochemical studies on megakaryocytes (MK) isolated from a Bernard-Soulier syndrome (BSS) patient whose giant platelets were characteristically deficient in GP Ib-IX complexes. Electron microscopy showed that the patient's MK were similar in size to normal MK. However, a striking feature was the variable and intermittent nature of the demarcation membrane system which was often vacuolar in appearance. Permeabilized mature MK from normal individuals were strongly positive with AP-2, AP-1 and FMC 25. Those from the BSS patient were normal for AP-2, negative for AP-1 but weakly positive with FMC 25. Binding of the monoclonal antibodies to the patient's platelets was evaluated using flow cytometry. The results confirmed the absence of GP Ib alpha from the surface membranes, but showed the presence of small amounts of GP IX distributed throughout the platelet population. Our findings confirm that the membrane lesion in BSS is also to be found in MK and further show that the defect may affect differently individual constituents of the GP-Ib-IX complex.