Disclosed is a process for determining the concentration of unbound hormone, e.g., thyroxine, cortisol, or testosterone, in a liquid sample containing hormone and protein capable of binding the hormone. The sample is incubated with antibody specific to the hormone to be detected and a distinguishable analogue of the hormone, both of which are separated from the sample by semipermeable membranes capable of excluding the passage of natural protein and antibody but which allow passage of the hormone and its analogue. Preferably, the antibody and analogue are contained in semipermeable microcapsules. Free hormone in the sample permeates the membranes and competes for sites of attachment to the antibody with the analogue. The antibody and free hormones are then separated. The amount of analogue present either in association with the antibody or in the remainder of the reaction system is indicative of the level of free hormone originally present in the sample.

The technique of separating a tagged bound material from the same unbound material in immunoassay procedures is improved by utilizing a clathrate sorbent to carry out the separation. A clathrate sorbent, for example, comprises a mixture of hydrate zinc sulfate and sodium hydroxide which form an inclusion compound with charcoal powder. Charcoal powder has also been combined to form a clathrate sorbent with salts of other metals which produce a gelatinous substance with alkaline hydroxides; e.g., alum with sodium hydroxide. Another example of a clathrate sorbent is magnesium trisilicate combined with the aforementioned zinc preparation.