Recently, Ochiaa et al.¹⁾ and Akiba et al.²⁾ found a remarkable phenomenon that multiple drug resistance (resistances to streptomycin, chloramphenicol, tetracycline and sulfonamide) of Shigella can be transferred, as a one-step event, into Escherichia coli by a conjugation process and vice versa. This finding was confirmed by Watanabe and Fukasawa³⁾, Mitsuhashi et al.⁴⁾ and Nakaya et al.⁵⁾ and it was suggested that the transfer is mediated by an episome.³⁾ In connection to this phenomenon, Mitsuhashi et al.⁶⁾ found a frequent co-existence of multi-resistant E. coli in patients bearing multi-resistant Shigella. The multi-resistant E. coli strains could be found at a relatively high rate in tuberculous patients who were administered with streptomycin, isoniazid and PAS (p-aminosalicylate). This report led the present writer to work as to whether the presence of PAS with chloramphenicol in intestine might cause a relatively rapid development o: chloramphenicol resistance by E. coli. This possibility was examined in in vitro experiments.