The main protein of the hemolymph of the cattle tick Boophilus microplus has been isolated and shown to be a heme lipoprotein (HeLp). HeLp has an apparent molecular mass of 354,000 and contains two apoproteins (103 and 92 kDa) found in equal amounts. HeLp presents a pI of 5.8 and a density of 1.28 g/ml and contains 33% lipids, containing both neutral lipids and phospholipids, and 3% of sugars. A remarkable feature of HeLp is the abundance of cholesterol ester (35% of total lipids), a lipid not previously reported in invertebrate lipoproteins. Western blot analysis showed HeLp in hemolymph from adult females and males, but not in eggs. Although HeLp contains 2 heme molecules, it is capable of binding 6 additional molecules of heme. Boophilus feeds large amount of blood, and we recently showed that this tick is unable to perform de novo synthesis of heme (Braz, G. R. C., Coelho, H. S. L., Masuda, H., and Oliveira, P. L. (1999) Curr. Biol. 9, 703-706). Injection of tick females with (55)Fe-labeled heme-HeLp indicated that this protein transports heme from hemolymph to tissues. HeLp is suggested to be an essential adaptation to the loss of the heme synthesis pathway.