Nalidixic acid inhibits the conjugal transfer of conjugative N incompatibility group plasmids.

The conjugal transfer of R plasmids of the N incompatibility group was inhibited by nalidixic acid to the same degree as that of plasmids of other conjugative and incompatibility groups. Inhibition was reversible. This supports a belief that nalidixic exerts its effect by interfering with a step(s) that is common to all conjugative systems among Enterobacteriaceae. These observations do not support an earlier suggestion that motivated the present study, and a reason for this is suggested.