Nonseptate filaments of Escherichia coli obtained by growth of a temperature-sensitive mutant of the organism at its restrictive temperature in the absence of antibiotics or at its permissive temperature in the presence of approximately one-half the MIC of penicillin (27 micrograms/ml; MIC of penicillin, 50 micrograms/ml) lacked the ability both to bind to mannose and to adhere to host tissues. Addition of low concentrations (0.5-10 micrograms/ml) of streptomycin to cultures of E. coli resulted in marked suppression of the mannose-binding and adhering ability of streptomycin-sensitive E. coli (MIC, 30 micrograms/ml). In contrast, up to 5,000 microgram of streptomycin/ml had no effect on an isogenic streptomycin-resistant mutant of E. coli (MIC, 20,000 micrograms/ml). No concentration of penicillin or streptomycin that was tested was able to suppress either the mannose-binding or the adhering ability of E. coli once those activities had been acquired by the organism. These results suggest that subminimal inhibitory concentrations of antibiotics suppress the ability of bacteria to adhere to cells.