The relative clearances of albumin, transferrin, haptoglobin, IgG, IgA, and IgM and the relative excretion of immunoglobin free lambda- and kappa light chains, lysozyme, and beta2-microglobulin were measured in 24 patients who had recently received renal transplants. Eleven out of 13 acute rejection episodes were predicted 1 to 5 days before the clinical diagnosis was made, and in 2 patients the clinical diagnosis of rejection and the prediction were made simultaneously. Successful antirejection therapy was followed by a decrease in protein excretion; in irreversible rejection episodes protein excretion continued to increase in spite of antirejection therapy. It is recommended that the relative clearances of IgG and haptoglobin be measured daily in patients recently treated by renal transplantation.