Red blood cell phospholipid arachidonic acid concentration was determined in 38 renal transplant recipients on cyclosporine-azathioprine-prednisolone therapy and in a comparable group of 20 patients on azathioprine-prednisolone alone. Samples also were obtained from 18 normal controls and 30 patients with "classical" hemolytic uremic syndrome (HUS). The arachidonic acid content was estimated as the percentage relative to the five principal fatty acids in red blood cell phospholipids (C16:0, C18:0, C18:1 omega 9, C18:2 omega 6, C18:3 omega 6) and quoted as mean value +/- standard deviation. There was a highly significant difference between patients on cyclosporine (14.7 +/- 2.9) and non-cyclosporine-treated transplant recipients (17.1 +/- 2.5; p less than 0.002). This difference was even more significant when patients who had been on cyclosporine for less than 3 months were excluded (14.1 +/- 2.7; p less than 0.001). The mean arachidonic acid content in non-cyclosporine recipients also was significantly less than that in normal controls (19.2 +/- 1.5; p less than 0.005) whilst the HUS patients (11.2 +/- 3.6) had significantly reduced values when compared with all the other groups. Cyclosporine often causes nephrotoxicity and in some cases HUS may develop in cyclosporine-treated transplant recipients. We have found a significant negative correlation between serum creatinine levels and red blood cell phospholipid arachidonic acid levels (r = -0.45; p less than 0.01). We propose that the decreased concentration of arachidonic acid in the cyclosporine-treated group may be related to the development of nephrotoxicity in the long term and may be a useful marker in predicting the early development of nephrotoxicity in these patients.