a1 Enteric Reference Laboratory, Public Health Laboratory Service, Colindale Avenue, London, N.W.9
The fi+ (fertility inhibition+) property was originally described in R factors related to F (F-like factors). The original fi− factors, in contrast, were related to the transfer factor of a colicin I factor (I-like factors). We describe here a group of R factors which are fi+ but I-like. It is suggested that all the terms fi+, fi−, F-like and I-like be used in the description of transferable plasmids.
(Received February 26 1971)