Shoot microcuttings of Malus domestica cv. Jork 9, Malus domestica cv. Cepiland, Mandevilla (Dipladenia) sanderi, and Daphne odora cv. variegata were induced to root by the addition of 0.1-1 mM dithiothreitol following an auxin (indole-3-acetic acid) treatment to initiate root formation. The most difficult to root, Malus domestica cv. Cepiland, required higher dithiothreitol concentrations to achieve optimum rooting than did the less difficult to root, Malus domestica cv. Jork 9, Mandevilla sanderi, or Daphne odora variegata. The roots induced by the combination of dithiothreitol and auxin treatment were approximately twofold longer and more robust than those induced with auxin treatment alone. The combination treatment of auxin and dithiothreitol has the potential to be of practical importance in root initiation in difficult-to-root cuttings of both woody and herbaceous horticultural varieties.