We initiated these studies to learn more about the initial events during bacterial conjugation and to optimize conditions for their occurrence. We found that cells in donor cultures grown anaerobically prior to mating have (i) a higher mean number of F pili per cell, (ii) longer F pili, (iii) a higher probability of forming specific pairs with F(-) cells, and (iv) a faster rate of initiation of chromosome transfer than cells grown aerobically. The growth medium for the donor culture also influences these same parameters: a rich medium is superior to a completely synthetic medium. Starvation of donor cells in buffered saline or for a required amino acid results in (i) a loss of F pili, (ii) a loss in the ability of donor-specific phages to adsorb, (iii) a loss of ability to form specific pairs with F(-) cells and to yield recombinants, and (iv) an increase in recipient ability. These changes occur as a function of starvation time, and at rates which are dependent on the conditions of prior growth and starvation of the donor culture. Either treatment provides a rapid method for the production of F(-) phenocopies from donor cultures. Resynthesis of F pili by cells within a starved donor culture commences very soon after restoration of normal growth conditions, but full restoration of donor ability, as measured by recombinant yield, occurs at a slower rate. We found, along with other investigators, that F pili are essential for specific pair formation. We also found, however, that the presence of F pili is not sufficient for display of donor ability, nor is the absence of F pili enough for cells to exhibit recipient ability. This suggests, therefore, that one or more components, in addition to F pili, are necessary for the conversion of specific pairs to effective pairs (or for chromosome mobilization, or both) and for preventing donor cells from acting as recipients. On the basis of our results, we suggest optimal conditions for achieving high mating efficiencies.