a method for determining the ambient concentrations of a plurality of analytes in a liquid sample of volume v liters , comprises loading a plurality of different binding agents , each being capable of binding specifically and reversibly an analyte of interest onto a support means at a plurality of spaced apart locations such that not more than 0 . 1 v / k moles of each binding agent are present at any location , where k liters / mole is the equilibrium constant of each such binding agent ; contracting the loaded support means with the sample to be analyzed , such that each of the spaced apart locations is contacted in the same operation with the sample , the amount of sample liquid being such that only an insignificant proportion of any analyte present in the sample becomes bound to the binding agent specific for it , and measuring a parameter representative of the fractional occupancy by the analytes of the binding agents at the spaced apart locations by a competitive or non - competitive assay technique , using a labelled site - recognition reagent for each binding agent capable of recognizing either the unfilled binding sites or the filled binding sites on the binding agent , which enables the amount of said reagent in the particular location to be measured . a device and kit for use in the method are also provided .