An invertase was purified from Cichorium intybus L. roots by a combination of ammonium sulphate precipitation, Concanavalin A affinity chromatography, anion exchange chromatography and gel filtration. A 230-fold purification and a final specific activity of 2.7 U per mg protein was obtained. The enzyme had a M(r) of 93 000 as estimated by gel filtration and after SDS-PAGE a single band with a M(r) of 50000 was found. Optimal activity was found between pH 5 and pH 5.5. At low substrate concentrations equal amounts of fructose and glucose were produced (apparent K(m) 55 mm). At higher sucrose concentrations progressively more isokestose was produced and fructose synthesis was inhibited. The high sucrose concentrations needed for isokestose synthesis do not support a physiological role for the enzyme in fructan biosynthesis.