A spectrophotometric study of the liver oils and vit. A concentrates of a series of Russian freshwater and marine fish was made. It was found that as a class, and with only rare exceptions, the liver oils of freshwater fish are differentiated from the analogous marine products by (a) their ultraviolet absorption spectra, and (b) the spectral absorption of their blue solns. in chloroformic antimony trichloride. Thus, the freshwater fish extracts exhibit an absorption maximum in alc. between 340 and 350 m[mu] (subsidiary maximum 285 m[mu]) compared with 325-328 m[mu] for marine fish liver oils. Similarly the freshwater fish liver oils give with SbCl3 in chloroform a blue colour exhibiting bands at 693 and 610-620 m[mu] with relative intensities (693/620 m[mu]) ranging from 0.5-2.5 (usually about 2.0). On the other hand, although the typical marine fish liver oils and vit. A concentrates contain both the 693 and 620 m[mu] chromogens, there is much less of the former than the latter (693/620 m[mu] ratio, order 0.15:1). From the mixed livers of Acipenser stellata, Silurus glanis, Abramis brama and Lucioperca lucioperca a product was obtained having E1cm 697 m[mu] = 2600, 620 m[mu]=1200, 344 m[mu] = 850, 285 m[mu] = 560. Attempts to separate the chromogenic substance responsible for the 697 m[mu] maximum from vit. A by either high vacuum distillation or chromatography, or to concentrate it further, have so far failed, but the results of careful ozonolysis have shown that the so-called "693 chromogen" is probably the C22 homologue of vit. A (C22H32O) with 6 ethylenic linkages. Comparative biological tests on Hippoglossus and Lucioperca concentrates of equal blue value indicate that the 693 m[mu] chromogen contributes to the total vit. A-activity as well as the 620 m[mu]. chromogen.