The effect of renal allograft rejection on the concentrations of the amyloid-related protein SAA, C-reactive protein (CRP), and beta 2-microglobulin (beta 2m) were compared, and the usefulness of post-transplant monitoring of these proteins in rejection diagnosis was evaluated. On the basis of the data from 30 reversible allograft rejections, the SAA test in rejection diagnosis was found to be more useful than either the beta 2m or CRP tests. The dramatic rejection-induced SAA elevation made it possible to choose a limit value for the SAA test that combined high sensitivity with reasonably high specificity. In contrast, limit values for beta 2m or CRP giving high sensitivity were combined with relatively low specificity. It is concluded that while post-transplant monitoring of SAA, CRP and beta 2m can all provide useful information aiding the early recognition and verification of acute allograft rejection, the properties of SAA are best suited for the purpose of a rejection marker.