The epoxides methyl 2,3-anhydro-beta-D-talopyranoside (1) and 2,6:3,4-dianhydro-1-deoxy-D-talo-hept-1-enitol (2), both prepared by improved methods, and 2,6:3,4-dianhydro-1-deoxy-D-gulo-hept-1-enitol (3) were applied as potential reagents for the affinity labelling of E. coli beta-D-galactosidase. Compounds 1 and 3 are ineffective as labelling reagents, whereas compound 2 irreversibly inhibits the enzyme activity. Deactivation is complete only when high concentrations (0.5M) of the inhibitor are applied over a relatively long period of time (24 h). Saturation kinetics cannot be observed. Nevertheless, the competitive inhibitor isopropyl 1-thio-beta-D-galactopyranoside protects the enzyme from irreversible deactivation by 2, indicating that the latter also reacts with the active site. Treatment of beta-D-galactosidase with 2,6:3,4-dianhydro-1-deoxy-D-talo-[4-3H]hept-1-enitol under conditions that effect deactivation of the enzyme to only a minor extent causes labelling of the protein at the molar ratio of 48:1. Specific, radioaffinity labelling of the active site of the enzyme cannot be thus achieved.