S U M M A R YWhen staphylococci, resistant to 4 pg. tetracycline/ml., were grown in nutrient media at subinhibitory levels of the drug, phenotypical resistance increased until the cocci grew with 160 pg. tetracyclinelml. Resistance increased most rapidly at the highest concentration of tetracycline which did not significantly inhibit growth. Increase in resistance was also obtained by pre-incubation with p-apo-5-oxy-tetracycline. Increase in resistance could be prevented by chloramphenicol and actinomycin D, but not by nalidixic acid. When a highly resistant culture was transferred to tetracycline-free medium, phenotypical resistance decreased gradually ; after four transfers on nutrient agar it returned entirely to the original level. I N T R O D U C T I O NWilliams (1967) has drawn attention to the resistance to mercury salts (Moore, I 960) of tetracycline-resistant strains of Staphylococcus aureus. Resistance to tetracycline was in general correlated with resistance to mercury; in mercury-sensitive strains, however, high-level resistance to tetracycline could be obtained by ' training ', i.e. growth in peptone water containing 3 pg. antibiotic/ml. METHODSBacterial strains. The strain studied, Staphjdococcus aureus I I I (Sompolinsky, Yiflah & Aboud, 1568), was susceptible to mercuiy salts (Moore, 1960) and resistant to tetracycline (Tc) by virtue of a plasmid that could be eliminated by growth at 44" (May, Houghton & Perret, 1964). The eliminated susceptible strain grew on nutrient agar containing 0.2 pg. Tc/ml. ; on agar containing 0.4 pg. Tc/ml., colonies did not develop from an inoculum of IO* staphylococci. With the uneliminated culture, identical colony counts were obtained on plain agar and on agar containing 4-0 pg. Tc/ml. At concentrations of up to 160pg. Tc/ml., some colonies developed but growth did not occur on nutrient agar containing 320pg. Tc/ml. Three types of Iesistant cultures have been used: (a) a culture transferred daily on plain nutrient agar; only 5 to 10% of the viable cocci from this culture produced colonies on agar containing 10 pg. Tc/ml. ; (b) a highly resistant culture subcultured daily on nutrient agar containing I 60 pg. Tc/ml.; and (c) cultures with intermediate levels of resistance obtained by growth of type (a) culture for a short period in nutrient broth containing tetracycline at different concentrations.