The probe specificity was evaluated by comparing the cleavage rates in the presence and absence of target. Targetdependent (specific) and target-independent (non-specific) cleavage rates of the probe were measured as the percent target cleaved per minute. The results are summarized in Table 1 . Probes I-III demonstrated similar specific cleavage. A 2-to 3-fold decrease in the rate of non-specific cleavage for probes II and III versus probe I was observed. These low, non-specific cleavage rates translate into high-specific to non-specific cleavage ratios. It appears that chloro-or chloro/ methyl substitutions in the fluorophores of probes II and III, respectively, are beneficial for the reduction of non-specific cleavage. One possible explanation for this reduction may be an effect of increased hydrophobicity of FL2 and FL3 on enzyme activity. In the case of probe IV, with the extended hydroxyprolinol linker, the rate of specific cleavage almost doubled. However, the corresponding increase in the rate of non-specific cleavage was $20 times higher than that observed in probe III, substantially compromising the ratio of specific to non-specific cleavage rates. Once again, this result implies the existence of inhibiting interactions between Endo IV enzymatic activity and fluorophore which can be adjusted by moving the fluorophore away from the active center.