Quaternary ammonium functionalized poly(propyleneimine) dendrimers were synthesized and their antibacterial properties were evaluated using a bioluminescence method. These quaternary ammonium dendrimers are very potent biocides. The antibacterial properties depend on the size of the dendrimer, the length of hydrophobic chains in the quaternary ammonium groups, and the counteranion. Since these dendrimers are well characterized and monodisperse, they also serve as an effective system to study the structure-activity relationship. The antimicrobial properties of these dendrimer biocides have a parabolic dependence on molecular weight, which is different from the bell-shaped molecular weight dependence of conventional polymer biocides. The dependence on the hydrophobic chain of the quaternary ammonium structure is similar to conventional polymer biocides, and shows a parabolic relationship with dendrimer biocides carrying C10 hydrophobes the most potent. The antimicrobial properties of these novel biocides with bromide anions are more potent than those with chloride anions. Biocides derived from hyperbranched polymers were also synthesized and found to possess somewhat lower effectiveness.