The conjugational transfer efficiency of 41 wild-type R-plasmids was studied in Escherichia coli K-12. Type I R-plasmids were transferred at comparatively high and rather uniform frequencies, whereas type F R-plasmids showed less uniform and, on average, somewhat lower transfer frequencies. R-plasmids not mediating sensitivity to F-, I-, or N-specific phages showed moderate transfer frequencies, and type N R-plasmids showed very low transfer frequencies. Various lines of evidence suggest that a well-expressed, but functionally inefficient, conjugation apparatus is the cause of the poor transfer of type N R-plasmids in liquid medium. Nalidixic acid efficiently inhibited transfer of type I and particularly type F R-plasmids, whereas the transfer of type N plasmids was resistant to the drug. Type F and type I plasmids appear to depend on at least one host function for their transfer, namely, the nalidixic acid-sensitive reaction in vegetative chromosome replication, whereas type N plasmids are independent of this function.