Two hundred and ninety-nine analyses of both blood lymphocyte count and lymphocyte ribonucleic acid/deoxyribonucleic acid (RNA/DNA) ratio in 17 patients with a renal allograft are reported. In the absence of infection, patients on standard immuno-suppressive regimes have normal results for the lymphocyte RNA/DNA ratio, although they may have lymphopaenia. Unequivocal infection and rejection may usually be distinguished by the lymphopaenia associated with rejection, although in both the lymphocyte RNA/DNA ratio is elevated. A clinically important distinction between lymphopaenia in rejection and lymphopaenia resulting from overimmunosuppression is made by measuring the lymphocyte RNA/DNA ratio. The ratio is low in oversuppression and elevated in rejection. The combined measurements of lymphocyte RNA/DNA ratio and lymphocyte count have a predictive value in more than one-half of the rejection episodes which show abnormality at least 5 days earlier than measurements which depend on renal function.