Urinary i-TXB2 of renal transplant patients was found to be unaffected by CsA administration. Thus CsA treatment is unlikely to contribute to false positive values in this diagnostic indicator of rejection. It is possible that CsA treatment may suppress the peak i-TXB2 attending rejection but this does not seem to be the case. Studies on rats with cardiac allografts show CsA not to attenuate the rejection i-TXB2 peak (8). The causal role of thromboxane in transplant rejection was further amplified by experimental studies in rats where the cardiac allograft was protected by thromboxane synthase inhibitors and receptor antagonists. The inflammatory cell infiltrate of clinical renal allografts correlated with urine i-TXB2. These data strengthens the diagnostic value of urine i-TXB2 as a non invasive indicator of transplant rejection. Serum gamma-interferon was studied in nine patients and detected for 14 and 10 consecutive days, respectively in 2 patients. Three of the remaining patients had mild rejection episodes. Of the two patients one rejected the kidney and the other had CMV infection. No correlation was found between gamma-interferon and urine-TXB2. Thus elevated serum gamma-interferon does not interfere with urine i-TXB2.