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A miniature optical biosensing system based on a PMOS phototransistor and absorption photometry is proposed. The phototransistor was manufactured in a standard 0.35-mum CMOS process, and it exhibited a responsivity higher than 1000 A/W for 650-nm light. For biochemical applications, the TMB/H2O2/HRP method was adopted as a useful basis in our system. A sample volume of only 10 mul was required to be dropped on the slide above the phototransistor. Experimental results demonstrated that a high sensitivity of 2.5 muA/pM was achieved, and the minimum HRP concentration successfully detected was 2.7 pM. This detection limit is three orders of magnitude better than that of a lately reported silicon biosensor, and is even comparable to that of a commercial spectrophotometer.