The photodynamic effect of 2 photosensitizers, phycOcyanin and CPD4, on colon cancer line HR8348, was measured with MTT assay. HR8348 cells were incubated in RPMI-1640 medium containing a series of concentrations of phycocyanin(100, 50 and 25μg/ml), and subsequently subjected to an irradiation of a pulsed copper vapour laser at the wavelength of 630 nm and a fluence of 12 J/cm2. The cell survival rates were 22.1%, 37.6% and 89.7%, respectively, (P< 0.001). The survival rates of cells treated with another photosensitizer, CPD4, at the concentrations of 10, 5 and 2.5 μg/ml, were otherwise 20.1%, 25.9% and 31.5%. MTT was a speedy and handy approach for screening photosensitizers.