For labelling of the metamorphosis hormone, ecdysone, with tritium, a modified Wilzbach technique has been worked out. Polyhydroxysteroids (Δ1-5α-Pregnendiol-(17α,21)-trion(3,11.20)-21-acetate and 5α-Pregnandiol-(17α,21)-trion(3,11,20)-21-acetate) were used as model substances. The best yields and highest activities were obtained by adsorbing the substances on granulated and activated charcoal, enclosing the absorbates in Nylon bags and exposing them to high doses of tritium at room temperature. The adsorption resulted also in a decrease of radioactive by-products. The tritiated substances were purified by thin-layer chromatography, using a special flow-counter for activity measurements on the chromatoplates. The method described is especially valuable with small amounts (5 - 20 mg). Specific activities up to 250 μC/mg of the pure substances have been obtained.