The object of this study has been to evaluate the recipient's immunological reactivity towards donor lymphocytes in relation to rejection episodes. All recipients (20) of local necrokidneys during 1976 were immunologically monitored immediately before transplantation and subsequently twice weekly for donor-specific complement dependent lymphocytotoxic (CDC) antibodies, antibody dependent cell-mediated cytotoxicity (ADCC) and cell-mediated lympholysis (CML). Experiments were performed until graft removal or dismissal (approx. 1100 patient days). Clinical diagnosis of rejection was made independently of immunological results. All clinically suspected rejection episodes, except one, were checked by microscopy. A positive CML-test accompanied 9 out of 11 rejection episodes; the test was negative on all other occasions. Positive CDC and ADCC tests exhibited no obvious correlation with rejection episodes: positive ADCC may be more frequent in clinically uncomplicated phases. Positive CML did not generally precede clinical graft rejection. Positive CML before transplantation was observed in two cases and was followed by irreversible, accelerated acute rejections. The CML-test may prove a reliable tool in rejection diagnosis and may yield results comparable with graft biopsy without inflicting any risk on the patient.