Eleven patients undergoing total hip replacement were studied by radioimmunoscintigraph for development of postoperative thrombus. Images were obtained using the 111In-labelled Fab' fragment of a monoclonal antibody (P256) directed against human platelets. Six patients showed focal accumulation of radioactivity in the lower limbs. Three of these had contrast venograms that were all positive for thrombus. Two of these six patients showed evidence of pulmonary localization of P256-Fab' and this correlated with positive ventilation/perfusion scintigraphy in one of the patients. Sequential imaging with 111In-P256 Fab' not only established a diagnosis of deep venous thrombus but also provided useful information regarding its natural history.