The majority of haemolymph proteins obtained from resting mature locusts are soluble in 50% saturated (NH4)2SO4 solution. Chromatographically, these proteins may be resolved on Sepharose 6B into three partially included fractions. The leading high molecular weight fraction, A, is predominant and all the diglyceride in “resting blood” is associated with it. An additional higher molecular weight fraction, A+, is present in the haemolymph of locusts injected with extracts of the glandular lobes of the corpora cardiaca (“elevated blood”), and this fraction carries all the increased diglyceride. Fraction A+ is better resolved on Ultrogel ACA 22. A useful rapid technique for studying the formation of A+ lipoprotein during lipid mobilization has been developed by the use of lipoproteinpolyanion-metal ion complex formation. A+ is soluble after heparin treatment whereas the yellow leading edge of fraction A precipitates. Concomitant with the appearance of A+ there is a marked depletion of a lower molecular weight non lipid-carrying protein fraction, C. The ability to form fraction A+ after injections of glandular lobe extract cannot be demonstrated in fledglings (immature locusts within 2 or 3 days after the imaginal moult) where fraction C is virtually absent. It is suggested that during lipid mobilization, some lipoprotein from fraction A combines with protein from fraction C to form A+. A direct effect of adipokinetic hormone on the process of A+ formation cannot be excluded.