Abstract: A biphase immunoassay method is provided in which a water immiscible liquid is qualitatively or quantitatively measured by mixing a sample of the water immiscible liquid with an aqueous solution containing a specific binding partner of the analyte. Binding occurs at or across the interface between the respective liquids and the degree of association of the analyte with its binding partner is dependent upon the concentration ratio rather than absolute quantities. The degree of association may be determined and used to determine the presence and/or concentration of analyte in the sample.