1. It was shown that a system previously described for labelling R-factor DNA during transfer to an irradiated recipient strain of Escherichia coli did not allow high selectivity in the incorporation of thymine into R-factor DNA. 2. Lack of selectivity was shown to be due to cross-feeding from recipient to donor strain. 3. An improved system using a nalidixic acid-resistant recipient strain is described in which incorporation of thymine into the DNA of donor cells is minimized by addition of nalidixic acid after completion of transfer of the plasmid during conjugation.

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