1) Of 18 strains of Escherichia coli, which acquired Streptomycin resistance in vitro, 4 strains acquired Sulfanilamide resistance simultaneously in contamination with Streptomycin. 2) But no Chlorampheuicol nor tetracycline resistant bacteria is observed. 3) 2 strains of the 18 Streptomycin resistaut strains, acquired severe Penicillin sensitivity. On the reasons, authors are researching now. It may be phenomenon of "Collateral Sensitivty (SZYBALSKI)".