Strain OU122, a dnaB mutant of HfrH which stops vegetative DNA synthesis immediately when the temperature is shifted to 43 degrees, was mated at 43 degrees with minicells with and without 1 mM Zn2+. Synthesis of DNA was detected in the mating mixture containing minicells derived from either Fchi925 or F+chi115 cells, but only a small amount was detected in the mixture containing 1 mM Zn2+ which inhibits the formation of mates (previously called mating pairs). The supernatant liquid from cell cultures did not induce DNA synthesis, suggesting that DNA synthesis was not stimulated by diffusible molecules. Additional experiments showed that Zn2+ inhibited DNA synthesis associated with DNA transfer but did not inhibit the DNA transfer that had already been initiated. Thus, the stimulation of DNA synthesis observed required physical interaction of cells and F pili. Reisolated minicell exconjugants derived from the cross OU122 X F--chi925 minicells were shown to contain transferred DNA in contrast to either the cross OU122 X F--chi925 minicells in 1 mM Zn2+, or the cross OU122 X F+chi1115 minicells. Thus, F+chi1115 minicells retained the property of surface exclusion at 43 degrees.