1. L.D. Hills, Comfrey, Past, Present and Future. Faber and Faber, London 1976.
2. H.E. Kirschner, Nature's Healing Grasses. H. C. White Publications, Riverside 1960.
3. L.B. Bull, C.C.J. Culvenor and A.T. Dick, The Pyrrolizidine Alkaloids. North-Holland, Amsterdam 1968, a) p. 234, b) p. 143, c) p. 163.
4. C.C.J. Culvenor and L.W. Smith, Aust. J. Chem.8, 556 (1955).
5. E. Pedersen, Arch. Pharm. Chem. sci. Ed.3, 55 (1975).