F plasmid won't transfer

[the End]

After "rescuing" the chlormaphenicol marked F plasmid from CJ236 by matings
to the ung- mutant BW310, I cannot get the plasmid to transfer to a 
third strain, MG4. MG4 is a "wild type" save for deletion in the lac operon
and a recA mutation. That is BW310F'(cmR) matings with MG4 fail to transfer the
marked F plasmid. 

Any ideas ?

Jim
J. Graham
.