No rapid and unequivocal test exists to diagnose renal allograft rejection. Despite advances in immunosuppression over the last 20 years, rejection is still the major problem in clinical organ transplantation. Most transplant centres report a renal graft survival rate of about 60 per cent at 5 years and the majority of graft losses are due to rejection. An influx into the graft of various cell types has been implicated in the diagnosis of rejection but it takes up to 24 h to obtain such information histologically. We have carried out fine needle aspiration biopsy of the allografts and estimated the number of white cells in the aspirate using a fluorescence activated cell sorter. Studies were performed in 34 cases of normal allograft function, 8 of rejection, 8 of acute tubular necrosis and 8 of cyclosporin nephrotoxicity. There was a statistically significantly higher cell number in the group undergoing rejection (P less than 0.01; Wilcoxon unpaired test) when compared to each of the other three groups. We believe that this technique, which takes only a few minutes to perform, could yield much valuable information. It deserves further study.