AbstractIt seems likely that the transfer of genetic material observed upon mating of male strains ofEscherichia coliK12 is due to (or is at least concomitant with) a particular replication of the donor DNA. The detailed mechanism of genetic transfer is discussed in the light of different, recently published experimental results.Although these data do not lead to a clear and definite conclusion at the present time, some points are now clearly established:(1) The DNA molecules transferred and recovered after mating from the recipient cells are synthesized during (and not before) mating; one strand of these DNA molecules is a parental DNA strand which comes from the parental donor cell.(2) The replication of the DNA seems to be required for transfer; it is not yet clearly established whether this replication has to take place in the male cells, in the female cells, or in both. However, some preliminary results suggest that a replication is necessary in the male cells only, and no clear evidence has been given to refute such an hypothesis.(3) One of the most interesting aspects of this problem is the identification of the mechanism which controls the transfer replication. The Hfr chromosome results from the fusion of two replicons (bacterial chromosome and sex factor), and an attractive hypothesis is that the chromosomal replicon controls the vegetative replication of the whole, and the sex factor controls transfer replication. Although this model has not yet been firmly demonstrated, it is supported by some arguments.