Another criterion for the presence of the agent (F) promoting genetic exchange in Escherichia coli was found. It involves a staining reaction of Hfr and F+ coli, but not F- coli, when mixed with strains of Salmonella typhimurium. This reaction was used as a guide in following the transfer of the F agent to Salmonella and back to E. coli. The F agent in Salmonella seems to promote the same kinds of events that it promotes in E. coli.