Photoinactivation and reversion to tryptophan prototrophy were studied in four Escherichia coli strains with different repair deficiencies. Cells were irradiated with 222-nm wavelength UV emitted by an excimer lamp and with 254-nm wavelength UV emitted by a low-pressure mercury lamp. Strain DSM 9494 (trp(-)uvrA(+)) turned out to be most resistant while the strain DSM 9495 (trp(-)uvrA(-)), which is defective in nucleotide-excision repair (NER) was most sensitive to both wavelengths. UV-fluence rates for a respective inactivation were twice as high for 222-nm wavelength UV than for 254-nm UV. No clear difference in efficiency of inactivation could be observed between the two wavelengths in strains DSM 9496 (trp(-)uvrA(+) pKM101) and DSM 9497 (trp(-)uvrA(-) pKM101). In general, more revertants were induced by 254-nm wavelength UV, which corroborates the hypothesis that a higher amount of DNA damage was induced by this wavelength than by 222-nm UV, except for DSM 9497 where no clear difference could be observed regarding the number of revertants induced by both wavelengths. This strain DSM 9497 has a high sensitivity to certain oxidative mutagens compared with other strains, which is indicative of formation of reactive oxygen species during irradiation with 222-nm wavelength UV.