An experiment was performed to elucidate the mechanism by which the noise letters exert their interfering effects on selective identification of a target letter. Ten undergraduate students were required to identify a target from a visual display of three letters arranged in a circle. The reaction time of pressing buttons to the target was measured under combined conditions of noise letters and assignment to response. The results supported the response level hypothesis that the noise letters exert their interfering effects, because they require the response incompatible with the target after having received processing by a feature analyzer. Further, it was indicated that the former results inconsistent with the response level hypothesis were artifacts caused by the selection of the stimulus letters.