We have examined the mutagenic properties in E. coli of single stranded vectors containing a uniquely placed cis-syn or trans-syn uracil-uracil cyclobutane dimer in the sequence 5' GCAAGUUGGAG 3', and compared these with the properties of the corresponding T-T dimers in the same sequence context. The frequencies with which U-U and T-T photoproducts were bypassed were similar in SOS induced cells, and each induced similar frequencies of mutations. However, although both U-U and T-T cis-syn dimers showed a preference for misincorporation in about 5-7% of the replication products, with T or G being incorporated in place of A, the ratios of these events differed, being > 4:1 for T-T cis-syn, but only 2:1 for U-U cis-syn. A shift towards G insertion opposite dimerized uracil was also found with the trans-syn dimers, but the difference was greater; T and G were misincorporated opposite the U-U trans-syn dimer in a ratio of 1:2, compared with 4:1 for its T-T counterpart. In addition, the U-U dimer induced only nucleotide substitutions, unlike the T-T photoproduct which induced single nucleotide deletions as well as substitutions. We conclude that even relatively minor differences in photoproduct structure, such as the presence of a methyl group at C-5, can alter mutational properties, and that such properties cannot depend only on the attributes of the DNA polymerase. Neither the efficiency of bypass, the error frequency nor the mutation spectrum of either U-U isomer is influenced by DNA uracil glycosylase. In vitro, the U-U cis-syn dimer is a substrate for DNA photolyase, but not for the glycosylase.