The cellular characteristics of three photodynamic sensitizers are described. The mechanism of uptake by cultured cells was determined for Photofrin II, Mono L-aspartyl Chlorin e6, and Chloroaluminum Sulfonated Phthalocyanine. In addition, the subcellular localization, sensitizer photostability and photochemistry were studied. Also, the integral relationship between the biological characteristics and the photochemistry of the sensitizers is discussed. Photofrin II enters the cells through passive diffusion across the plasma membrane and primarily localizes in the mitochondria. Both the chlorin and phthalocyanine are transported into the cell through constitutive pinocytosis and thus localize in the lysosomes.