A strategy for radioimmunoassay of plasma progesterone with use of a homologous-site 125I-labeled radioligand.

In this sensitive, specific 125I radioimmunoassay for plasma progesterone, an identical glucuronide bridge is used in the immunogen and radioligand. This apparently provides a system in which the bridge is poorly recognized and thereby avoids the excessively high affinity of the tracer for the antiserum and the consequent poor sensitivity that commonly is found in other homologous-bridge situations. The glucuronide bridge is potentially useful for attachment of other labeling probes.