Factors have been identified which are responsible for the phenomenon already reported in which regrowth of Pseudomonas aeruginosa was found to occur in broth cultures containing bactericidal concentrations of carbenicillin. In cultures incubated under stationary conditions, in a water bath, the factor primarily responsible for this phenomenon of regrowth appears to be the formation of condensate on the inside of the culture vessel. In such condensate, viable cells of P. aeruginosa were found in numbers equal or higher than those in the culture broth. As a result of coalescing, and running down the vessel wall, the condensate provides a continuous reintroduction of bacteria into the culture medium, and, notwithstanding a bactericidal concentration of antibiotic in the medium, this process of continual reinoculation leads to the formation of visible growth and an increase in the viable count in the culture medium after an initial bactericidal effect. In silicone-treated flasks, incubated in a water bath, regrowth did not occur. Under such conditions, condensate again formed, but only as discrete droplets on a non-wetting surface, and in such condensate the bacterial count was found to be low. In flasks incubated in an air incubator, condensate did not form, and under these conditions the phenomenon of regrowth was not observed.