Abstract: A heterogeneous binding assay in which a liquid component and a granular particulate solid phase are incubated together for a predetermined period of time. To prevent unwanted unit gravity sedimentation of the granular particulate solid phase during incubation the density of the liquid component is controlled such that it is substantially equal to the density of the granular particulate solid phase, thus preventing sedimentation. The density may be controlled by adding a density modifying medium such as a colloidal suspension of silica particles coated with polyvinylpyrrolidine. After incubation, the density of the liquid component may be reduced allowing unit gravity sedimentation and hence separation of the solid phase from the liquid component. The assay is particularly applicable to immunoassay (for example radioimmunoassay and immunoradiometric assay). Immunoradiometric assays for human growth hormone, thyroid stimulating hormone and alphafetoprotein are described.