Modern diving techniques can require the treatment of infection in an atmosphere of pressurized oxyhelium gas. The antibiotic susceptibility of 16 species and strains (eight genera) of gram-negative bacilli and 3 species and strains (two genera) of gram-positive cocci to each of 21 antimicrobial agents was assessed in air at atmospheric pressure and in oxyhelium gas at an absolute pressure of 7 bar (ca. 709 kPa). A disk diffusion technique was employed, and significantly different results were obtained in the two atmospheres. The effect of oxyhelium on diameters of growth inhibition varied significantly with the bacterium and with the antibiotic and was particularly marked with certain bacterium-antibiotic combinations. The gram-negative bacilli generally gave reduced zone diameters in oxyhelium, whereas the gram-positive cocci showed a mixture of effects.