Senecio longilobus Benth. (thread-leaf groundsel) is considered to be one of the most poisonous of the groundsels, especially to cattle and horses, although it has been used extensively in the domestic medicine of the Indians (Kearney and Peebles, 1951). Apparently the leaves of new growth are most toxic and the primary effect is to produce lesions in the liver. Other species in the genus are less poisonous but are capable of developing cumulative effects when ingested by livestock in small doses over extended periods of time. The alkaloids present in several of the toxic species have been isolated, but not all of the Senecio species contain alkaloids, nor are all of the alkaloids toxic to livestock. Manske (1931) has reviewed some of the literature on the subject and proposed the generic name necine for the basic hydrolytic product and necic acid for the acid fragment.