Abstract: A process and apparatus has been developed for radioassay of ligand in solution which eliminates the separation step required in conventional techniques. A chamber is provided containing a quenching solution, a plurality of ligand molecules and a plurality of receptor molecules. One of pluralities forms a free species labelled with a beta particle emitter while the other is immobilized on a solid support, e.g., the chamber wall or a microbead, within the chamber. Ligand introduced with the sample competes with ligand molecules already in the chamber for receptor sites on the receptor molecules and the free species is allowed to diffuse about the chamber. A beta particle detector in communication with the chamber at a fixed position detects only those beta particles emitted from within the quenching distance of the quenching solution. The quenching properties of the solution are used in place of the conventional separation step.