An assay for assessing the proteolysis of IGFBP-3 by a biological fluid or protein has been developed using 125IhIGFBP-3 recombinantly derived from Escherichia coli as a substrate indicator. The labelled substrate is incubated at 37 degrees in the presence of the agent being tested and proteolysis is visualized by SDS-PAGE and autoradiography. The assay showed that the pregnancy associated protease is Ca++ dependent. The substrate is also proteolyzed by rat and mouse pregnancy sera, making the assay applicable to studies in those animal systems.