Six of seven lactose-fermenting (lac(+)) Salmonella strains obtained from clinical sources were found to be capable of transferring the lac(+) property by conjugation to Salmonella typhosa WR4204. All of the six S. typhosa strains which received the lac(+) property transferred it in turn to S. typhimurium WR5000 at the high frequencies typical of extrachromosomal F-merogenotes. These six lac elements were also transmissible from S. typhosa WR4204 to Proteus mirabilis and to some strains of Escherichia coli K-12; moreover, they were capable of promoting low frequency transfer of chromosomal genes from S. typhimurium WR5000 to S. typhosa WR4204. One of these lac elements was shown also to be capable of promoting low frequency chromosome transfer in E. coli K-12. E. coli K-12 strains harboring these lac elements exhibited sensitivity to the male specific phage R-17. Sensitivity to R-17 was not detected in Salmonella strains containing the elements. Examination of the lac elements in P. mirabilis by cesium chloride density gradient centrifugation showed that each element had a guanine plus cytosine content of 50%. The sizes of the elements varied from 0.8 to 3% of the total Proteus deoxyribonucleic acid. The amount of beta-galactosidase produced by induced and uninduced cultures of S. typhimurium WR5000 and S. typhosa WR4204 containing the lac elements was lower than that produced by these strains with the F-lac episome. The heat sensitivity of beta-galactosidase produced by the lac elements in their original Salmonella hosts indicated that the enzyme made by these strains differs from E. coli beta-galactosidase.