The major alkaloids in Senecio mikanioides Otto are sarracine and its N-oxide. Sarracine is shown to be platynecine esterified on the 1-hydroxymethyl group with 2-hydroxymethyl-2-butenoic acid (sarracinic acid, structure VI) and on the 7-hydroxyl group with angelic acid. Angelic acid amide was also isolated from the plant, but this may be an artefact. The common occurrence of this plant as a creeper on waste land near Melbourne prompted its reinvestigation with the aim of securing sufficient "mikanoidine" to determine whether it possessed hepatotoxic activity. Physical properties and composition of sarracine and its N-oxide agree with those reported for the alkaloid sarracine and its N-oxide isolated by Danilova et al. from S. sarracenius, and their identity was confirmed by the preparation of derivatives and hydrolysis results. The structure of sarracinic acid was established as VI via NMR spectrum and other data, showing it is attached to the primary hydroxyl group of platynecine. The configuration of the double bond in sarracinic acid was determined to be the same as that of angelic acid using τ values of the olefinic proton.