SUMMARY: Pyronin Y (0.06 and 0.16 mM), coumarin (2 and 4 mM), 6,9-dimethyl 2-methyl-thiopurine (2 mM) and caffeine (8 mM), strongly inhibit Uvr repair (presumed excision repair) of u.v. lesions in Escherichia coli. With 0.16 mM-pyronin Y or 4 mM-coumarin, one thymine dimer was a lethal event in the K12 strain JC 2926 rec A-13 and one to three dimers in the B/r derivative WP-2 rec A-13 and the B exr A derivative BS-2. Neither these compounds nor 8 mM-caffeine had a significant effect on the survival of irradiated bacteria of the corresponding uvr strains WP-2 Hcr-, BS-12, and BS-8. I suggest that 8 mM-caffeine, 0.16 mM-pyronin Y, 4 mM-coumarin and 2 mM-6,9-dimethyl 2-methylthiopurine selectively block excision repair without substantially affecting recombination repair. Caffeine at 12 mM only slightly depressed the viability of a u.v.-irradiated culture of WP-2 Hcr- or BS-12, but at 16 mM there was a much greater effect, particularly at low u.v. doses (< 40 ergs/mm2). Since excision repair appeared to be effectively blocked by 8 mM-caffeine, the depression of u.v. survival of the uvr mutants by 16 mM-caffeine was presumed to be due to interference with recombination repair (Witkin & Farquharson, 1969).