Abstract Strains of Escherichia coli ‘K-12’, sensitive or resistant to chloramphenicol, proflavine or brilliant green were crossed, and the drug resistance of the recombinant strains examined. For this purpose a quantitative scale of resistance was established, so that the spectrum of the distribution among the recombinants could be determined. In general, the recombinants showed a continuous spectrum or resistances in the complete range between the values for the two parent strains, with a greater probability of values towards the middle of the range. When both parents were resistant no recombinant showed resistance appreciably greater than the higher of the parental values. Parent strains proved to be of two types (probably identifiable with the donors and receptors of other workers) such that the cross Aresistant x Bsensitive gave a nearly complete recovery of parental units of resistance, while the cross Asensitive x Bresistant gave a much smaller recovery. The interpretation of these results is discussed in general terms.