The efficiency of recovery of P1 transductants is marker dependent and normally varies over a 25-fold range. UV irradiation of either transducing lysates for recipient cells results in a selective stimulation of the transduction of markers which are normally transduced poorly. As a result the range in frequency of transduction is reduced to about 3-fold and resembles the gene frequency distribution expected in the donor cells. We conclude that P1 transducing lysates are likely to contain a random sample of donor DNA but that the recombination system of the recipient cell exhibits a preference for the DNA of some regions over that of others. Damage to DNA presumably overrides this specificity.