Reported in this paper are experiments conducted to compare formaldehyde, glyoxal, malondialdehyde, succindialdehyde, glutardialdehyde, adipindialdehyde, beta-propiolacton, and pyrocarbonic acid diethylester for their toxifying properties. Tetanus toxin, 1.5 million dlm/ml or 120 binding units, was used. Glutardialdehyde proved to be the most favourable compound and was followed by formaldehyde against the background of the experimental arrangement used, that is determination of concentrations for inactivation and antigenic properties of the toxoids. Glyoxal, malondialdehyde, succindialdehyde, and beta-propiolactone followed in some distance, whereas no inactivation at all was obtained from adipindialdehyde and pyrocarbonic acid diethylester.