In vivo studies of the incorporation of radiolabeled precursors demonstrate that proteins of the high molecular weight fraction (HMWF) are synthesized during the time that the embryo is free and unattached in the uterus prior to implantation.

Progesterone was found to be responsible for regulating the secretion of HMWF components in long term ovariectomized rabbits. Estradiol-l7 if administered simultaneously with progesterone, however, had an inhibitory effect on progesterone induction of protein synthesis.

Immunological studies demonstrate that there are at least 4 antigens present in the HMWF and absorption studies indicate that at least 2 of these are uterine specific antigens.

The HMWF can be separated into 3 subfractions by Sepharose 6B chromatography. The first of these contains components with molecular weights greater than 4 x 10⁶ and in the second the components range in molecular weight to 100,000. The third fraction is a lower molecular weight entity (MW <100,000) which is separated from the heavier components during chromatography. This fraction does not appear to be protein and gives a positive orcinol reaction.

Step gradient polyacrylamide gel electrophoresis demonstrates the presence of 2 major proteins in the HMWF from Sephadex G-200 chromatography. Sodium dodecyl sulfate electrophoresis resolves as many as 17 polypeptide chains, 4 of which incorporate labeled amino acids in vivo.