Plasma mebendazole levels were analysed retrospectively in patients treated for inoperable infections with Echinococcus multilocularis or granulosus. In 10 patients receiving mebendazole at 4 dose levels there was no relation between dose and plasma concentration. In 17 patients followed on the same dose for more than 18 months, the plasma levels varied with individual coefficients of variation ranging from 27 to 72%. The data reveal the limitations of single measurements of plasma mebendazole and emphasize the need for repeated monitoring. Coadministration of phenytoin and carbamazepine seemed to lower plasma levels, presumably as a result of enzyme induction. It was not possible appreciably to raise the mebendazole concentrations by inhibition of drug metabolizing enzymes with cimetidine.