We have serially measured serum beta 2 M microglobulin in a series of transplant recipients along with other standard clinical parameters. Independent comparison of the beta 2 M results leads to the following conclusions: 1. Beta 2 M is superior to the Scr in detecting acute rejection, with diagnostic elevations occurring 2 to 7 days before Scr increase. The observation is valid for all rejection episodes. 2. Beta 2 M decreases prior to or simultaneously with the Scr following successful rejection therapy or beginning resolution of acute tubular necrosis. 3. Abnormal beta 2 M following rejection therapy invariably heralds another rejection episode within 10-20 days, despite the Scr having returned to baseline. 4. Beta 2 M remains normal in high grade ureteral obstruction despite increased Scr. 5. Beta 2 M is remarkably increased in patients with viremia, despite minimal change in Scr. Beta 2 M remains normal in lower UTI from bacterial origin. Beta 2 M appears to be a major contribution in the monitoring of the renal transplant recipient which may have significant impact on therapeutic decisions in the future. In addition, it provides a reliable in vitro parameter which can be used to further assess specific treatment variables in a prospective controlled protocol approach.