SUMMARYSalmonella typhimurium tolerated the replacement of about 70 % of the thymine in the DNA by 5-bromouracil (BU). In phage PLT-22 transduction, the presence of BU in the donor DNA had no effect on the linkage relationship of two contransduced markers and did not conclusively alter the ratio of complete transductants to abortive transductants. The yield of transductants per plaque-forming unit was more than doubled by the treatment of the donor bacteria and phage with BU. However, more than half of the BU-treated phage particles, detected by electron microscopy, did not form plaques; when the multiplicity of infection was calculated from the titre of physical particles, the frequency of transductants was normal. It is concluded that recombination was not affected by the substantial incorporation of BU into the donor DNA. Treatment of the recipient bacteria with BU slightly modified recombination by decreasing the frequency of transductants by about 40 % and the linkage of the cotransduced markers by about 5 %. Because comparable results were not obtained when the donor DNA contained BU, it is suggested that these effects resulted indirectly from the treatment of the recipient bacteria.