Indian Institute of Experimental Medicine, Calcutta-32 Chemistry Department, Bethune College, Calcutta-6 Manuscript received 24 February 1975; accepted 19 April 1975 Two fatty acids have been isolated from the oil of Argemone mexicana seeds. From the study of mass spectral fragmentation pattern, IR and NMR spectra coupled with various chemical reactions these two acids have been characterised as 11-oxo-triacontanoic acid (Ib) and 11-hydroxy triacontanoic acid (Ib). The properties of the former appear to be identical with those of the recently reported uncharacterised compound designated as sanguinarine potentiating factor (SPF) isolated from the oil and the latter is a new acid isolated for the first time from a natural source.