We investigated the effect of chain length on the bactericidal activity of benzalkonium chloride during a short contact time by counting survivors. The C8 and C10homologues had very weak bactericidal activity. The bactericidal concentrations of the C12 and C14 homologues were 6-400μg/ml at 10 min of contact at 25°C against 15 strains of test bacteria. The C16 and C18homologues showed variable bactericidal activity towards the 15 strains tested.As the carbon chain was lengthened, the killing rate decreased and inhibition of the bactericidal activity by organic materials increased. The bactericidal concentrations of the C12 homologue at 1 min of contact at 25°C were 5000μg/ml in a suspension of 2.5% dried yeast, and 2500μg/ml in a 10% solution of human serum. Those of the C16 homologue were more than 10000μg/ml in both cases.We assayed unbound benzalkonium chloride in solutions of bovine serum albumin by high-performance liquid chromatography and found that the bactericidal activity in a solution of bovine serum albumin arose from it.These results show that from a practical point of view, C12-benzalkonium chloride is the most effective component of the homologues of benzalkonium chloride.