Immunoscintigraphy with technetium-99m labelled BW 250/183, a murine monoclonal antibody specific for granulocytes, yielded a false-positive result in a patient suspected of having an abscess in his renal graft. To substantiate the presumption that diathesis and unspecific accumulation of the antibody may have caused this result, ten selected patients were investigated who presented with chronic vascular graft rejection but without signs of bacterial infection. Scintiscans were recorded 4 and 24h after administration of 99mTc-labelled BW 250/183. Graft-background ratios (GBRs) were calculated for each transplant. These were compared with the mean of physiological kidney-background ratios (KBRs) and with bone marrow-background ratios (BMBRs). After removal, the grafts were examined with pathological and immunohistological methods. Seven transplants demonstrated 4-h GBRs (mean: 3.9+/-1.1, P <0.001) significantly outside the range of normal KBRs while three were within the normal range (mean: 1.8+/-0.4). The relation between 4-h and 24-h GBRs varied. After 24h five GBRs still remained increased (mean: 3.2+/-1.4, P <0.05). By contrast the BMBRs decreased uniformly by 18%+/-5%. After graft removal, histopathology demonstrated no dominant granulocyte accumulations but various degrees of chronic vascular and tubulo-interstitial rejection. Immunohistochemical studies did not indicate cross-reactivity of BW 250/183. Increased GBRs of long-standing renal allografts indicate the passage of the antibody through injured vascular walls rather than the presence of granulocyte accumulations. Therefore, variability of GBRs with time reflects changes in transitory concentrations of 99mTc-labelled BW 250/183 in the tissues.