Abstract
Using the Hfr (high frequency of recombination) strain of Hayes, derived from strain K-12, results are obtained which are consistent with the idea of oriented transfer (Jacob and Wollman) but also indicate the occurrence of postzygotic elimination (Lederberg). When ability to synthesize methionine, rather than resistance to streptomycin, is used as the F" (recipient mating type) selective marker, and the cross mixture is incubated for 2 hours, oriented transfer is not apparent.