The material employed for this investigation consisted of the fresh seeds of Casimiroa edulis, La Llave and Lejarza (Nat. Ord. Rutaceae), which were obtained directly from Mexico. For the purpose of their examination, the kernels of the seed were first separated from the dry, fibrous shells. The latter, which represented about one-fifth the weight of the entire fresh seed, were separately examined, but yielded little of interest. They were found to contain a small amount of an essential oil, traces of alkaloidal substance, some sugar, and considerable stearic acid in the free state, together with resinous material. The kernels of the seed, when subjected to a preliminary test, gave reactions indicating the presence of a considerable proportion of an alkaloid. They were also found to contain an enzyme, which was obtained in the form of a light brown powder, and slowly effected the hydrolysis of amygdalin. An alcoholic extract of the ground kernels, when distilled in a current of steam, yielded a small amount of a pale yellow essential oil, which gradually darkened in colour. This oil had an agreeable, aromatic odour, and the following constants: d=0.9574 at 20°; αD + 2°25′ in a 25 mm. tube. From the portion of the extract which was soluble in water there were isolated: (i) a new alkaloid, casimiroine, C24H20O8N2 (m. p. 196-197°), which is a very weak base, but yields a crystalline picrate (m. p. 165°) and an aurichloride (m. p. 195-196°). On heating casimiroine with alkalis, it undergoes hydrolysis, with the elimination of carbon dioxide, yielding a new base, casimiroitine, C23H22O7N2 (m. p. 171°), together with a small amount of a basic substance melting at 159°; (ii) a new alkaloid, casimiroedine, C17H24O5N2 (m. p. 222-223°; [α]D, in acid solution, -36.5°), which yields a crystalline aurichloride; (iii) benzoic acid, with apparently a trace of salicylic acid. The aqueous liquid contained, furthermore, a quantity of sugar, which yielded d-phenylglucosazone (m. p. 205-207°). The portion of the alcoholic extract which was insoluble in water consisted of a soft, oily resin. From this material the following compounds were isolated: (i) sitosterol, C27H46O (m. p. 134-135° [α]D -32.1°), (ii) ipuranol, C23H38O2(OH)2; (iii) a mixture of fatty acids, consisting of palmitic, stearic, oleic, linolic, and linolenic acids; (iv) a new lactone, casimirolid, C24H28O6 (m. p. 229-230°; [α]D -49.2°), which yields a new hydroxy-acid, dasignated as casimiroic acid, C23H28O4(OH)·CO2H (m. p. 207°; [α]D -86.8°) The following derivatives of this acid were prepared: silver casimiroate, C23H29O5·CO2Ag; methyl casimiroate, C23H29O5·CO2CH3 (m. p. 108-110°); and acetylcasimiroic acid, C23H28O4(O·COCH 3)·CO2H; (v) a yellow, phenolic substance, C16H12O6 (m. p. 215-218°). It may finally be noted that the results of the present investigation of Casimiroa seed have afforded no evidence of the presence of a definite glucoside or a so-called "glucoalkaloid," as has previously been affirmed, and physiological tests conducted with animals have likewise failed to confirm their reputed hypnotic or toxic properties.