The human placental receptor (alpha 2MR) for alpha 2-macroglobulin-proteinase complexes contains 3 polypeptides of approx. 500 kDa, 85 kDa, and 40 kDa. N-terminal sequence analysis of the 500 kDa and 85 kDa polypeptides, analysis of a random selection of peptides convering 536 residues from these polypeptides, and analysis of a 1772 bp cDNA encoding part of the 500 kDa polypeptide provide evidence that the 500 kDa and 85 kDa chains are the alpha- and beta-subunits, respectively, of a recently cloned hepatic membrane protein, termed the low density lipoprotein receptor related protein (LRP) (Herz, J., Hamann, U., Rogne, S., Myklebost, O., Gausepohl, H. and Stanley, K.K. (1988) EMBO J. 7, 4119-4127; Herz, J., Kowal, R.C., Goldstein, J.L. and Brown, M.S. (1990) EMBO J. 9, 1769-1776). N-terminal sequence analysis of the 40 kDa polypeptide shows that it is of distinct genetic origin. It is suggested that LRP is the functional receptor for alpha 2-macroglobulin-proteinase complexes (alpha 2MR) and in addition may have as yet unsettled functions in lipoprotein metabolism.