Glucuronidation studies using microsomes and recombinant uridine diphosphoglucuronosyltransferases (UGTs) can be complicated by the presence of endogenous beta-glucuronidases, leading to underestimation of glucuronide formation rates. Saccharolactone is the most frequently used beta-glucuronidase inhibitor, although it is not clear whether this reagent should be added routinely to glucuronidation incubations. Here we have determined the effect of saccharolactone on eight different UGT probe activities using pooled human liver microsomes (pHLMs) and recombinant UGTs (rUGTs). Despite the use of buffered incubation solutions, it was necessary to adjust the pH of saccharolactone solutions to avoid effects (enhancement or inhibition) of lowered pH on UGT activity. Saccharolactone at concentrations ranging from 1 to 20 mM did not enhance any of the glucuronidation activities evaluated that could be considered consistent with inhibition of beta-glucuronidase. However, for most activities, higher saccharolactone concentrations resulted in a modest degree of inhibition. The greatest inhibitory effect was observed for glucuronidation of 5-hydroxytryptamine and estradiol by pHLMs, with a 35% decrease at 20 mM saccharolactone concentration. Endogenous beta-glucuronidase activities were also measured using various human tissue microsomes and rUGTs with estradiol-3-glucuronide and estradiol-17-glucuronide as substrates. Glucuronide hydrolysis was observed for pHLMs, lung microsomes and insect-cell expressed rUGTs, but not for kidney, intestinal or human embryonic kidney HEK293 microsomes. However, the extent of hydrolysis was relatively small, representing only 9-19% of the glucuronide formation rate measured in the same preparations. Consequently, these data do not support the routine inclusion of saccharolactone in glucuronidation incubations. If saccharolactone is used, concentrations should be titrated to achieve activity enhancement without inhibition.