A method and apparatus for the determination of trace amounts of chemicals in a system, as for example the immunoassay determination of hormones, peptides and drugs in biological fluids, applies photoelectrochemistry to the field of immunoassay. The apparatus and method comprise a photoelectrochemically active molecule as a label in an immunoassay system of labeled analyte, antibody sensitive to analyte, quencher and an electrochemical flow cell with light means. The photoelectrochemically active molecule upon photoexcitation transfers an electron to a quencher molecule; the oxidized molecule is subsequently reduced with an electron from an electrode of the flow cell which is held at suitable potential. This electron is measured as photocurrent. The electrochemical flow cell is channeled for the flow of solution containing photoelectrochemically active species and quencher; and has at least one wall transparent to light such as a laser which is trained on the appropriate region of the cell; and further comprises means to isolate the spectral region of interest and means to distinguish photocurrent from nonphotocurrent. The amount of free labeled analyte in the system is determined by the photocurrent signal.