The lysine residues in thaumatin I were chemically modified by acetylation with acetic anhydride and by reductive methylation, under various conditions. The acetylated and methylated thaumatins were isolated by ion-exchange chromatography. The number of remaining free amino groups was determined by trinitrophenylation. At least four acetylated thaumatins with either one, two, three or four acetylated amino groups were obtained as well as one methylated thaumatin with six dimethyl lysine residues and one monomethyl lysine residue. The sweetness intensity of the acetylated thaumatins decreased with the increasing number of acetylated amino groups; the sweet taste had disappeared completely when four amino groups were acetylated. The methylated thaumatin with seven modified lysine residues had a sweetness intensity practically equal to that of the original thaumatin. The total net change, i.e. the isoelectric point of thaumatin, might play a role in the physiological behaviour of thaumatin causing a sweet taste sensation.