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Placental lactogen, a protein secreted by the human trophoblast, shows marked similarities to human growthhormone in immunological properties (Josimovich & MacLaren, 1962), in amino acid composition (Friesen, 1965) and sequence (Catt, Moffat & Niall, 1967a; Sherwood, 1967) and in the possession of prolactin-like activity (Josimovich & MacLaren, 1962; Forsyth & Folley, 1968). In contrast, molecular-weight determinations for the two hormones suggest that the growth hormone exists as monomer whereas the placental lactogen has a dimeric structure. For the growth hormone, molecular weights of 21500 obtained in the ultracentrifuge (Li & Starman, 1964) and 20500 by gel filtration (Andrews, 1966), both without the use of dissociating solvents, are close to the value required for a single-chain structure (Li, Liu & Dixon, 1966). On the other hand, sedimentation analysis of the placental lactogen gave molecular weights of 38 000 in a non-dissociating solvent and 18500 in 5Mguanidine hydrochloride, suggesting that the hormone molecule consists of two polypeptide chains of equal weight held together by hydrogen bonds (Florini et al. 1966). The results of Catt, Moffat, Niall & Preston (1967b) also imply a dimeric structure, but a molecular weight of30 000 estimated by the gel-filtration method (Friesen, 1965) suggests the possibility of a ready dissociation. Investigation of the gel-filtration behaviour of human placental lactogen now indicates that it does, in fact, exist in dilute solution as a monomeric form similar to that of human growth hormone. Materiak and methods. Purified human placental lactogen was generously provided by Dr P. H. Bell, Lederle Laboratories, Pearl River, N.Y., U.S.A. (lot no. 4508C-75), Dr K. Catt, Department of Medicine, Monash University, Melbourne, Vic., Australia (lot no. 37), Dr H. Friesen, Royal Victoria Hospital, Montreal, Que., Canada, and Dr M. M. Grumbach, School of Medicine, University of California, San Francisco, Calif., U.S.A. All four preparations were assayed for prolactin-like activity by Dr Isabel A. Forsyth, with both the rabbit mammary intraductal test (Chadwick, 1963) and a local method of pigeon crop-sac assay (Rivera, Forsyth & Folley, 1967). Activities in the rabbit test were about 24i.u./mg. in each case, and in the pigeon assay only 2-4i.u./mg. (Forsyth & Folley, 1968). Gel filtration was performed at 2-5° with a colunm (50cm. x 2-3cm. diam.) of Sephadex G-100 [Pharmacia (G.B.) Ltd., London W.13] equilibrated with 5mM-tris-HCl buffer, pH7 5, containing

Molecular weight of human placental lactogen investigated by gel filtration