This paper describes a detailed study of Tumour necrosis factor release during the immediate post transplant period in recipients of Liver and Kidney allografts. The results show no correlation between levels of TNF and rejection in Azathioprine and steroid treated patients and give good correlation in cyclosporin immunosuppressed patients. Treatment with Campath 1-G caused high levels of TNF production. Interleukin-1 (IL-1) levels were not associated with levels of TNF. Segmental measurement of TNF levels may provide an aid to early diagnosis of graft rejection.