A method for determining a biological response of a target (41, 42) to a soluble candidate substance comprises the steps: introducing a soluble candidate substance into a laminar flow of a buffer liquid (2) to form a candidate substance solute (3) having an initial concentration profile (31); dispersing the initial concentration profile (31) to form a dispersed concentration profile (32); directing the dispersed concentration profile (32) into a detection channel (12) to form a final symmetrical concentration profile (33) therein; introducing a target into the detection channel (12) to obtain a combined concentration profile comprising a constant target concentration profile overlying the final symmetrical concentration profile (33); holding in the detection channel (12) at least one half of the combined concentration profile; and optically scanning the combined concentration profile to detect an optical signal representative of the biological response of the target to the soluble candidate substance.