Investigations have been reported on the use of neopterin and soluble interleukin-2 receptors as rejection markers. Each of these analytes is available as part of an easy-to-perform analytic method. Each is available in a diagnostic kit that is accurate, precise, and sensitive enough to provide good clinical information. Both of these materials have a significant relationship with the cell-mediated immune response, particularly at the level of T-cell activation. Neopterin does increase significantly in cases of transplant rejection but shows greater response to viral infection. Soluble interleukin-2 receptors increase during periods of transplant rejection but also respond during T-cell activation brought on by bacterial or viral infection. The use of these markers is best described for kidney transplants and less well documented for liver and heart transplants.