The effect of various concentrations of natural and chemically modified cyclodextrins on the luminescence of an Escherichia coli suspension was investigated. All cyclodextrins were found to reduce, albeit to a varying degree, the luminescence level of the bacterial cells, thus suggesting a direct interaction between the cyclodextrins and cells. The inhibitory concentrations IC20 and IC50 of the various cyclodextrins were determined and taken to represent their toxicity effects upon the bacterial cells. Among the natural cyclodextrins, gamma- and alpha-CD interfered minimally with the bacterial luminescence and consequently were essentially non-toxic. The following descending order of toxicity was observed : beta-CD much greater than alpha-CD > gamma-CD. Among the chemically modified cyclodextrins, Dimeb was clearly toxic while Trimeb and the hydroxylated derivatives (hydroxypropyl-alpha-CD, HPACD; -beta-CD, HPBCD; -gamma-CD, HPGCD) were essentially non-toxic. The following descending order of toxicity was observed : Dimeb much greater than HPBCD > Trimeb > HPACD > HPGCD.