Abstract: A method for determining the concentration of the free fraction of an active compound, present in a biological fluid, in the presence of natural binders, the free and bound fractions of the active compound being in mutual equilibrium, bya) contacting a sample of the fluid with an unlabeled antibody,b) separating the sample from the unlabeled antibody,c) incubating the unlabeled antibody with a labeled substance (tracer) for cross-reaction with the antibody andd) measuring the amount of the tracer which is or is not bound to the antibody and calculating from this the concentration of the free fraction of the active compound,wherein the quantity of the unlabeled antibody and/or its affinity for the active compound are so small that they do not substantially effect the equilibrium between the free and bound fractions of the active compound, and the affinity of the tracer for the antibody is substantially higher or substantially lower than that of the active compound itself, and a test kit suitable for this metho

Abstract: A method for determining the concentration of the free fraction of an active compound, present in a biological fluid, in the presence of natural binders, the free and bound fractions of the active compound being in mutual equilibrium, bya) contacting a sample of the fluid with an unlabeled antibody,b) separating the sample from the unlabeled antibody,c) incubating the unlabeled antibody with a labeled substance (tracer) for cross-reaction with the antibody andd) measuring the amount of the tracer which is or is not bound to the antibody and calculating from this the concentration of the free fraction of the active compound,wherein the quantity of the unlabeled antibody and/or its affinity for the active compound are so small that they do not substantially effect the equilibrium between the free and bound fractions of the active compound, and the affinity of the tracer for the antibody is substantially higher or substantially lower than that of the active compound itself, and a test kit suitable for this metho

Abstract: The invention relates to a process for the stabilization of biologically active substances which are immobilized on a carrier, where the solid phase with the immobilized biologically active substance is contacted for the stabilization with a solution which contains polyanetholesulfonic acid and/or salts thereof. The biologically active substances are, in particular, antibodies.

Abstract: The invention relates to novel thyronine derivatives of the formula ##STR1## in which n is 10-400, R denotes H, alkyl or N-carbonylthyronine, R.sup.1 and R.sup.2 are identical or different and denote hydrogen or iodine, and R.sup.3 denotes H or alkyl, a process for the preparation of the compounds, and their use when carrying out immunoassays.

Abstract: The invention relates to new thyronine derivatives of the formula ##STR1## in which n is 1-6, R denotes hydroxyl, alkyl or aryl, R.sup.1 and R.sup.2 are identical or different and denote iodine or hydrogen, and R.sup.3 denotes hydrogen, alkyl or aralkyl, a process for the preparation thereof, and the use thereof for carrying out radioimmunoassays.