Experiments have been performed to test the validity of a model which relates low recovery of very early donor markers in recombinants to the presence of sex-factor deoxyribonucleic acid at the leading end of donor deoxyribonucleic acid transferred by conjugation. This model has been found to be incorrect. The results obtained do not rule out the possibility that a piece of sex factor is transferred at the leading end, but indicate, however, that if this occurs it is not the cause of the low recovery of a very early donor marker.