A miniature CMOS optoelectronic biosensor is proposed for screening whitening agents. Based on the principle of absorption photometry, this biosensor is mainly composed of a green LED, a CMOS phototransistor, and a slide for performing the biochemical reactions. The requirement of the sample volume is only 10 μl, much less than those in commercial instruments. Melanin assay and MTT assay were performed to evaluate the whitening mechanisms. Experimental results demonstrated that the density of 5x104 cells/ml can be successfully detected for both assays. This portable biosensor has great potential toward personalized diagnostic kits for further applications.