In Serratia marcescens and Pseudomonas aeruginosa from clinical specimen acetyltransferase AAC-(6') IV leads to varying phaenotypically expressed resistance to aminoglycoside antibiotics. The substrate profile and the rate of inactivation of the enzymes investigated are almost identical with each other and with the enzyme of P. aeruginosa GN 315. The enzymes cannot be associated with R factors by the standard methods for conjugation, transformation, density gradient centrifugation and agarose gel electrophoresis.