The stability of dactimycin to aminoglycoside-modifying enzymes produced by 341 bacterial clinical isolates has been studied. Enzymatic activities were measured by the phosphocellulose binding assay. The results demonstrated that dactimicin was stable to the following enzymes: (i) AAC(3)-II,-III,-IV and -V. (ii) AAC(2'); (iii)AAC(6')-I and -II;(iv) ANT(2"); (v)ANT(4'); (vi) APH(3')-I,-II,-III and -IV. In contrast, dactimicin was only inactivated by two enzymes, AAC(3)-I and the bifunctional AAC(6')/APH(2"). This staphylococcal enzyme modified and inactivated dactimicin by acetylation but not by phosphorylation, suggesting the possibility of a second target amino group, such as 6'-NH2, in addition to the C4 amino group, which is the target for AAC(3)-I.