Accurate detection of a genetic or biochemical marker introduced into sugarbeet (Beta vulgarjs L.) is based on the absence of native sequences or activities in the plant that could confound the analysis of expression of the introduced marke r. During the course of experiments designed to optimize DNA transfer from Agrobacterjum tumef aciens to sugarbeet leaf disc cells, an endogenous enzyme activity was discovered which utilizes all the common substrates recognized by the marker enzyme l3,-glucuronidase (GUS) from E. coli. This native sugarbeet enzyme (SB-GUS) was characterized immunologically and biochemically. GUS and SB-GUS were found to be distinct with regard to pH optima, thermal inactivation, reaction to denaturants and protein modifying reagents, inhibition by metals and saccharo-Iactone, and molecular mass. The two activities are not immunologically related, as judged by Western blot and immunoprecipitation analyses. A protocol was developed to accurately quantitate introduced GUS in the presence of SB-GUS, by utilizing selective inhibition of GUS at pH 7.0 by saccharic acid l,4-lactone. Under these conditions GUS activity is completely eliminated, while SB­ GUS activity was unaffected.