[en] A comparison of the frequency of trp+ revertants of Escherichia coli B/r Hcr+ thy trp after UV irradiation on the one hand and after UV irradiation plus photoreactivation on the other showed that both photoreversible pyrimidine dimers of the cyclobutane type and the non-photoreversible DNA lesions cause, at equal lethal effects, also trp+ reversions with the same efficiency. If lethal, the pyrimidine dimers may thus be conceived as primary pre-mutational lesions. (author)