This study compares the usefulness of serum and urine fibrin split products and the urinary enzyme, beta-glucuronidase, in the diagnosis and management of renal transplant rejection. Fibrin split products, determined by a tanned human red cell agglutination inhibition immunoassay, were measured as a reflection of the secondary fibrinolysis from fibrin deposited in the renal microvasculature as a result of rejection. Urinary beta-glucuronidase, expressed as the ratio of enzyme activity to creatinine concentration, was determined by a colorimetric technique following dialysis of urine to remove endogenous activators and inhibitors. Activity of this lysosomal enzyme is thought to reflect tubular injury. Twenty-nine renal transplant recipients (15 from living donors and 14 from cadaver donors) were evaluated. Both serum and urinary fibrin split products and urinary beta-glucuronidase were markedly elevated in the immediate postoperative period, probably reflecting ischemic trauma. Acute rejection occurring within the first three months was associated with elevations of fibrin split products (particularly urine) and beta-glucuronidase. Elevated values returned to normal following successful treatment with steroids and/or heparin, but remained high in the presence of continued rejection. After the first 48 hours post-transplant, in the absence of rejection, values for fibrin split products were within the normal range. Urinary beta-glucuronidase remained elevated if the transplanted kidney was recovering from acute tubular necrosis. Fibrin split products and urinary beta-glucuronidase were usually normal in chronic rejection.