SUMMARY Plasmids containing the genetic determinants for resistance to tetracyclines (Tc), streptomycin (Sm) and sulphonamide (Su), colicine production (Col) and a-haemolysin production (Hly) were transferred from four wild strains of Escherichia coli to E. coli K 12 by conjugation brought about by F factor. Linkage was demonstrated between F and Tc and between Sm and Su. No linkage was noted between Col, Hly, F and Tc, and Sm and Su, which were transmitted as four independent units in no predetermined order. F and Tc were eliminated together during acridine orange and sodium lauryl sulphate treatment ; the other characters were not eliminated. After mating for pro- longed periods, F and Tc were most frequently found in the recipients. Recipients of other characters in the absence of F and Tc did not transmit them.