Samuelsson, G. (Dept. of Pharmacognosy, Faculty of Pharmacy, Lindhagensgatan 128, S-112 51 Stockholm, Sweden) 1974. Mistletoe toxins. Syst. Zool. 22:566–569.—The European mistletoe, Viscum album L, contains several small proteins which are toxic to mammals on parenteral administration. Screening of 49 different species of mistletoes representing 9 genera, has shown the presence of such proteins in 9 species from 5 genera, all within the subfamily Viscoideae. The amino acid sequence has been determined for three proteins from Viscum album and one protein from Phoradendron tomentosum subsp. macro-phyllum. These proteins all contain 46 amino acid residues with 3 disulfide bridges. The sequences are identical in 29 positions. 11 differences can be explained in terms of a single base replacement in the coding triplet. For 7 differences two base replacements must be assumed.