Phthalocyanine (Pc) dyes are a class of photosensitizers that are being considered for use in the photodynamic therapy of cancer. A final choice for the most useful drug demands phototoxicity screening of various structurally related Pc dyes, such as those with different substituents at the peripheral benzene rings. Using the colony forming ability of Chinese hamster cells as an endpoint, the photobiological activity for ZnPc was found to be the highest for the unsubstituted compound, followed in decreasing order by the 2,9,16,23-tetrahydroxy-, 2,9,16,23-tetrasulfonated-, and 2,9,16,23-tetraneopentoxy-Pc derivatives. This effect could be correlated with the uptake rate of the respective Pc derivatives by the cells, but it is unrelated to the apparent lipophilicity of these compounds, or, alternatively, to their ability to photogenerate singlet oxygen. The first synthesis of a metal-free tetrahydroxy Pc is described.