A method and apparatus for determining qualitatively or quantitatively the presence of analyte bound to a separation media without doing a bound/free separation. In the method, the bound fraction is collected in an assay region of a body of liquid which includes the free analyte, and the assay is performed by comparing the radiant-energy response in the assay region to the radiant-energy response in a control region of the body of liquid which is free of bound analyte. The apparatus has a chamber which contains the body of liquid, one or more collection elements and a control element and position in the body of liquid parallel to an opaque wall which has a colliminating slit in registry with each element. Each slit enables sensing of the radiant-energy response from the body of liquid between the slit and its associated elements.