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Extracts of single hypophyses of the finback whale (Balaenoptera physalus) were exposed to zone electrophoresis on starch, and the electrophoretic samples tested for their melanophore-stimulating activity, using pieces of isolated skin of the lizard Anolis carolinensis in vitro as a test object. It was demonstrated that four different components with melanophore-stimulating activities are present in a single whale hypophysis. The electrophoretic behaviour of three of these components are comparable to that of the MSH's of known chemical structures alpha-, beta-seryl-, and beta-glutamyl-MSH. The electrophoretic behaviour of a fourth component, differs from that of any of the known MSH's. This component is the most abundant – or has the strongest melanophore-stimulating activity – in comparison with the other components found in the whale. The possibility that this component is identical with the fat mobilizing factor discovered and isolated by Birk & Li (1963) is suggested.