The effect of the antibiotics gentamicin, streptomycin, kanamycin, tetracycline, and ampicillin on planktonic cultures of Enterobacter aerogenes, Serratia marcescens, Salmonella derby, Streptococcus mitis, and Staphylococcus epidermidis with and without an application of 70 kHz ultrasound was studied. The ultrasound was applied at levels that had no inhibitory effect on planktonic cultures of bacteria. Measurements of viability at, above, and below the minimum inhibitory concentration of the above antibiotics on the planktonic cultures of these bacteria showed that a simultaneous application of 70 kHz ultrasound and antibiotic significantly increased the effectiveness of selected antibiotics. Bacterial viability was reduced several orders of magnitude when harmless levels of ultrasound were combined with some antibiotics, especially the aminoglycosides. Similar synergistic effects of combined ultrasound and antibiotic treatment were seen in both Gram-positive and Gram-negative bacteria with several classes of antibiotics. These results may have application in the treatment of bacterial infections normally resistant to some antibiotics.