DNA strand breaks, measured by alkaline elution, and hypoxanthine guanine phosphoribosyltransferase (HPRT) mutation were studied in V79 cells after photochemical treatment (PCT) or exposure to X-rays. Cells were incubated with the photosensitizers Photofrin II (PII) and three closely related porphyrins tetra-(3-hydroxyphenyl) porphyrin (3THPP), meso-tetra-(4-sulfonatophenyl) porphine (TPPS 4) and meso-tetra-( N-methyl-4-pyridyl) porphine (TMPyPH 2). These dyes are assumed to act on cellular targets mainly via singlet oxygen when excited by light. While the hydrophilic TPPS 4 and TMPyPH 2 did not photoinduce mutants to any significant extent, both lipophilic dyes, 3THPP and PII, were significantly mutagenic when excited by light. On the other hand, TPPS 4 was the most efficient sensitizer of alkali-labile DNA strand breaks, while TMPyPH 2 did not induce any significant amount of either type of DNA damage. Surprisingly, no correlation between the two parameters was found for PCT, either after exposures inactivating 50% of the cells or after exposures inactivating 90% of them. The lack of correlation between the yields of DNA strand breaks and of mutants could not be explained by differences in the intracellular localization pattern of the dyes.