Biomaterials that inactivate microbes are needed to eliminate medical device infections. We investigate here the antimicrobial nature of single-walled carbon nanotubes (SWNTs) incorporated within the biomedical polymer poly(lactic-co-glycolic acid) (PLGA). We find Escherichia coli and Staphylococcus epidermidis viability and metabolic activity to be significantly diminished in the presence of SWNT–PLGA, and to correlate with SWNT length and concentration (<2% by weight). Up to 98% of bacteria die within one hour on SWNT–PLGA versus 15–20% on pure PLGA. Shorter SWNTs are more toxic, possibly due to increased density of open tube ends. This study demonstrates the potential usefulness of SWNT–PLGA as an antimicrobial biomaterial.