The present invention provides a method of determining the presence or activity of a clostridial toxin by (a) treating with a sample, under conditions suitable for clostridial toxin protease activity, a clostridial toxin substrate which includes a fluorophore; a bulking group; and a clostridial toxin recognition sequence containing a cleavage site that intervenes between the fluorophore and the bulking group; (b) exciting the fluorophore with plane polarized light; and (c) determining fluorescence polarization of the treated substrate relative to a control substrate, where a change in fluorescence polarization of the treated substrate as compared to fluorescence polarization of the control substrate is indicative of the presence or activity of the clostridial toxin.