THE presence of a phenyl group enababes penicillin G to be estimated in mixtures containing other penicillins of non-aromatic type. The spectra were photographed on Ilford Auto filter plates with a Hilger medium quartz spectrograph (E 498) using the continuous radiation from a hydrogen discharge lamp ; an exposure of 30 sec. was needed with a slit width of approximately 0.05 mm. Under these conditions, phenyl absorption bands at 2685, 2645, 2585 and 2525 A. are observed with a solution of sodium penicillin G in a mixture (1 : 30) of water and ethanol having a concentration of 15 mgm./ml. at a path-length of 1.5 mm.