Abciximab, chimaeric Fab fragments of the monoclonal antibody 7E3 (c7E3 Fab), has achieved widespread use as an anti-platelet agent for blocking GP IIb-IIIa (alphaIIbbeta3) function and preventing ischaemic complications after coronary artery angioplasty. However, its accessibility to the bone marrow compartment during therapy is unknown, as is its ability to bind alphavbeta3 in vivo. Using electron microscopy and immunogold labelling, we have looked for abciximab in the bone marrow of a patient who became thrombocytopenic during treatment. The presence of abciximab was assessed on ultrathin frozen sections of a marrow aspirate, the drug being revealed by a rabbit antibody to c7E3 Fab. Labelling was maximal on fragmenting megakaryocytes (MK) and proplatelets in the vascular sinus and in direct access to the blood compartment. Not only the plasma membrane but also the demarcation membrane system (DMS) and the membranes of alpha-granules were labelled. Abciximab was also revealed on the luminal surface of endothelial cells lining the marrow sinuses, thereby confirming for the first time its ability to bind to alphavbeta3 in vivo. The study revealed no signs that abciximab had accumulated in the marrow.