The activities of tobramycin derivatives acetylated and ethylated on the 6'-
N,
2
'-
N and
3
-
N positions were examined. The MICs of these derivatives against tobramycin sensitive strains indicated that
2
'-
N-ethylated and 6'-
N-ethylated derivatives have a fairly good activity, and confirmed that the
3
-
N position is the most important one for antibiotic activity since
3
-
N derivatives were less active. The MICs of these derivatives against tobramycin resistant strains, and their inactivation by tobramycin modifying enzymes were examined. These results showed that
2
'-
N or 6'-
N ethylation protects the drug against inactivation by AAC(
2
') or AAC(6'), respectively, and
2
'-
N-ethyltobramycin and 6'-
N-ethyltobramycin were active against strains containing these modifying enzymes. On the other hand,
3
-
N ethylation protects the drug against inactivation by AAC(
3
) but
3
-
N-ethyl tobramycin does not inhibit strains containing this enzyme.
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