A number of poisonous plants have a considerable influence on the milk yield of animals which eat them, reducing it in volume or in fat content, or imparting to it an unpleasant flavour which renders it unfit for human consumption, and even affecting the butter made from it. Other plants not actually poisonous also affect milk by “tainting” it, and must therefore be considered harmful otherwise than as mere weeds. Still others affect milk by causing it to be stained with blood, and in some cases the toxic principle of the plant concerned is even stated to have rendered the milk poisonous. It will be useful to give here a brief account of milk-affecting plants, though it is probable that there may be others which are not mentioned. The plants best known in this connection are the Garlics (_Allium oleraceum_ L., _A. ursinum_ L., _A. vineale_ L., and others), which impart to milk a strong “oniony” flavour. These plants also seriously affect the flesh of animals which eat them, even rendering it unfit for consumption. A case came before the author several years ago in which a considerable number of sheep had eaten a quantity of _Allium ursinum_, which was growing in a field in which they had been grazing for some days. The meat was so strongly tainted that, as the owner (a butcher) stated, it was quite unfit for sale. The only thing to be done was to pasture the sheep elsewhere for a time. Pott notes that species of _Ranunculus_ give rise to reddish or bitter milk (_Mölkereizeitung_, 1897), while _Caltha palustris_ causes loss of milk production in cows. _Ranunculus repens_ imparts a strong unpleasant flavour to the milk of cows that have eaten it, and the butter made from such milk is distinctly bitter in taste (Güssow). _Ranunculus sceleratus_ causes a falling-off in milk-yield. (Cornevin.) _Alliaria officinalis_ Andrz, imparts an oniony flavour to milk. _Achillea Millefolium_ L. is stated to impart its bitter taste and strong odour to dairy products when eaten by cows. It contains an alkaloid, _Achilleine_ (C_{20}H_{38}O_{15}N_{2}) having a peculiar odour and bitter taste. _Chrysanthemum Leucanthemum_ has also been suspected of imparting a bad flavour to butter. According to Pott the milk of cows eating _Oxalis Acetosella_ is with difficulty converted into butter. The poisonous principle of _Colchicum autumnale_ is stated to find its way into the milk of animals consuming it, and Müller states that the milk of goats which have eaten it has caused the poisoning of infants. Müller states that a cat died after drinking the milk of a cow suffering from poisoning by _Narthecium ossifragum_. _Mercurialis annua_ is said to cause milk to be thin, “blue,” and poor in fat; while in a case already mentioned (p. 68) _M. perennis_ entirely stopped the secretion of milk.