Donor Hfr CS101 has gene transfer curves displaying two distinct parts; an initial low slope which breaks sharply into a second steep slope. We have established that the first slope represents the true time of gene entry into receptor cells. Under similar conditions of mating, donor Hfr C shows classical linear transfer curves with no marked break in the slope. When chromosome transfer was followed in specific mating pairs formed five minutes after contact, it was found that although both Hfr CS101 and Hfr C gave rise to equal numbers of effective contacts, Hfr C gave rise to from 10 to 30 times the number of recombinants as Hfr CS101. Also, Hfr C has a transfer delay of between 15 and 20 minutes from the time of gene entry till plateau, while Hfr CS101 has a transfer delay of from 40 to 50 minutes. Therefore, these kinetic transfer results of Hfr CS101 appear to be the result of a unique alteration of chromosome mobilization and transfer. Transfer of genetic markers has been followed in the donor Hfr C which gives rise to classical kinetics and in the unusual donor in the presence and absence of acridine orange, a dye previously found to inhibit chromosome transfer during conjugation. At a concentration of 30 μg acridine orange/ml. in the mating broth, transfer of the markers by Hfr C is inhibited by about 50% but the unusual transfer of Hfr CS101 is unaffected. We have isolated the sex factor segment from Hfr CS101 and tested the autonomous F′ factor's sensitivity to curing by acridine orange. As controls, strains A237 and P678-F8 carrying autonomous F′ factors were also subjected to acridine orange treatment after the method of Hirota. Following growth in broth containing 50 μg acridine orange/ml., strains A327 and P678-F8 had lost their autonomous F′ factors but strain P678-3 still retained its autonomous F′ factor isolated from Hfr CS101. Therefore, in contrast to all prior reports in the literature with other sex factors, the replicating mechanism of the sex factor segment derived from Hfr CS101 is resistant to curing by acridine orange.