Abstract: The present invention is directed to the measurement of soluble T cell growth factor receptors, soluble T cell differentiation antigens, or related soluble molecules or fragments thereof, and the use of such measurements in the diagnosis and therapy of diseases and disorders. The measurement of such molecules can be valuable in monitoring the effect of a therapeutic treatment on a subject, detecting and/or staging a disease in a subject, and in differential diagnosis of a physiological condition in a subject. These measurements can also aid in predicting therapeutic outcome and in evaluating and monitoring the immune status of patients.In specific embodiments, measurements of serum or plasma interleukin-2 receptor levels can be made, to detect or stage leukemia or lymphoma. In other embodiments, IL2R levels, or CD8 levels, can be used to differentially diagnose renal allograft rejection, as distinguished from Cyclosporin A nephrotoxicity.