The perfusion of kidneys with anti-CD45 monoclonal antibodies prior to transplantation offers a means of targeting passenger antigen-presenting cells with the aim of reducing the subsequent incidence of rejection episodes. A safety study was performed in humans of such pretreatment in 40 unsensitized recipients of first cadaveric renal grafts, who were followed for 3 months after transplantation. A 50-ml solution containing 2 mg of each of the rat anti-CD45 mAbs YTH 24.5 and YTH 54.12 was injected into the allograft renal artery ex vivo and just before transplantation while the renal vein was kept clamped. No patients died, but 4 grafts were lost. Two were lost due to primary nonfunction, 1 was lost because of late renal artery thrombosis, and 1 was lost to rejection. There were no cases of renal vein thrombosis and 1 trivial renal artery stenosis, and only 2 patients produced human anti-rat antibodies. Between 63.5% and 100% (median 96.4%) of CD45+ cells in the postperfusion biopsies were coated with anti-CD45 as determined by double-immunolabeling. The number of patients experiencing rejection episodes was inversely associated with this "antibody uptake": 75% of the low uptake group (< 95%) had at least 1 rejection episode, compared with 22% of the high uptake group (> or = 95%) (P = 0.001). The complement components C3 and C5b-9 colocalized with perfused anti-CD45 in 32/33 (97.0%) and 11/33 (33.3%) of the biopsy specimens, respectively. We conclude that: (1) this technique appears free of adverse effects, (2) high antibody uptake within the kidney is associated with a lower incidence of rejection, and (3) the antibodies used fix and activate complement in vivo.