The effectiveness of surfactants as potentiators of antibiotic activity on several resistant strains of bacteria, selected from clinical sources and laboratory collections, was studied using a tube dilution assay. Bacterial strains included members of the Enterobacteriaceae and staphylococci. Cetyltrimethylammonium bromide (CTAB), Tween 80 (Tw80), a mixture of n -alkyldimethyl betaines (L14), and alpha-(2,4,5-trichlorophenoxy) propionic acid (TCP) were tested in combination with pencillin G (PenG), methicillin (Met), streptomycin (Sm), polymyxin B (PmB), and chlortetracycline (CTC). Growth response to the drug combinations was compared with the response to each drug alone. CTAB and L14 but not Tw80 or TCP were found to potentiate the activity of CTC on strains of Escherichia coli, Proteus mirabilis , and Klebsiella pneumoniae . Studies on the inhibition of protein synthesis by CTC in cells of a strain of E. coli suggested that the surfactants increased the uptake of antibiotic into the cells. CTAB and L14 almost completely sensitized strains of P. mirabilis, Serratia marcescens, K. pneumoniae , and E. coli to PmB. With the exception of K. pneumoniae , TCP was also effective in potentiating the activity of PmB on the above strains whereas Tw80 showed potentiation only with a strain of E. coli . CTAB and L14 but not TCP or Tw80 potentiated the activity of PenG but not Met on strains of staphylococci. Studies of penicillinase in the cells suggested that the surfactants inhibited the formation of this enzyme possibly at the level of induction. None of the surfactants were found to potentiate the activity of Sm.