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 .