The action of atomic hydrogen on trypsin (EC126.96.36.199) in aqueous solution has been investigated at different initial concentrations, total doses of H atoms and pH. Concurrent with the inactivation of the enzyme, specific changes in the disulphide bonds, ultraviolet absorption spectrum and fluorescence spectrum have been observed. The attack on the disulphide bonds results in the formation of sulphydryl groups and of hydrogen sulphide. The observed decrease of the 280-mμ absorption peak is consistent with the assumption that tryptophan residues are affected and this is supported by the fluorescence data. The pH dependence shows a minimum effect at about pH3. Rate constants for enzyme inactivation, formation of sulphydryl groups, formation of hydrogen sulphide and destruction of tryptophan have been calculated.