A method of conjugating a saccharide to a protein carrier using carbodiimide condensation chemistry, wherein the saccharide comprises (for instance as part of its repeating unit), or has been derivatised to comprise, amino and/or carboxyl groups, and wherein the protein carrier comprises, or has been derivatised to comprise, amino and/or carboxyl groups, comprising the steps of: I) - if the protein carrier comprises both amino and carboxyl groups and the saccharide comprises either amino or carboxyl groups: a) mixing the saccharide and aliquot of carbodiimide required to perform the conjugation, and b) adding the aliquot of protein carrier required over a period of 5 minutes to 6 hours, wherein at least one quarter of the aliquot is added over the first half of the period, and at least one quarter of the aliquot over the second half of the period; II) - if the saccharide comprises both amino and carboxyl groups and the protein carrier comprises either amino or carboxyl groups: a) mixing the protein carrier and aliquot of carbodiimide required to perform the conjugation, and b) adding the aliquot of saccharide required over a period of 1 minute to 6 hours, wherein at least one quarter of the aliquot is added over the first half of the period, and at least one quarter of the aliquot over the second half of the period; III) - if the saccharide comprises both amino and carboxyl groups and the protein carrier comprises both amino and carboxyl groups: a) mixing the protein carrier and saccharide, and b) adding the aliquot of carbodiimide required to perform the conjugation over a period of 1 minute to 6 hours, wherein at least one quarter of the aliquot is added over the first half of the period, and at least one quarter of the aliquot over the second half of the period; and wherein: the aliquot of carbodiimide is 0.01 to 3 mg carbodiimide / mg saccharide; the saccharide is present at a final concentration of 0.5-50 mg/ml in step b); the protein carrier is present at a final concentration of 1-50 mg/ml in step b); the initial ratio of protein carrier to saccharide is 4:1 to 1:1 (w/w); the reaction pH in step b) is maintained at pH 4.5-6.5, or pH 4.5-7.5 if a compound is present in step b) which keeps the reaction intermediate stable; the temperature of the reaction in step b) is maintained at 4-37°C. I) - if the protein carrier comprises both amino and carboxyl groups and the saccharide comprises either amino or carboxyl groups: a) mixing the saccharide and aliquot of carbodiimide required to perform the conjugation, and b) adding the aliquot of protein carrier required over a period of 5 minutes to 6 hours, wherein at least one quarter of the aliquot is added over the first half of the period, and at least one quarter of the aliquot over the second half of the period; a) mixing the saccharide and aliquot of carbodiimide required to perform the conjugation, and b) adding the aliquot of protein carrier required over a period of 5 minutes to 6 hours, wherein at least one quarter of the aliquot is added over the first half of the period, and at least one quarter of the aliquot over the second half of the period; II) - if the saccharide comprises both amino and carboxyl groups and the protein carrier comprises either amino or carboxyl groups: a) mixing the protein carrier and aliquot of carbodiimide required to perform the conjugation, and b) adding the aliquot of saccharide required over a period of 1 minute to 6 hours, wherein at least one quarter of the aliquot is added over the first half of the period, and at least one quarter of the aliquot over the second half of the period; a) mixing the protein carrier and aliquot of carbodiimide required to perform the conjugation, and b) adding the aliquot of saccharide required over a period of 1 minute to 6 hours, wherein at least one quarter of the aliquot is added over the first half of the period, and at least one quarter of the aliquot over the second half of the period; III) - if the saccharide comprises both amino and carboxyl groups and the protein carrier comprises both amino and carboxyl groups: a) mixing the protein carrier and saccharide, and b) adding the aliquot of carbodiimide required to perform the conjugation over a period of 1 minute to 6 hours, wherein at least one quarter of the aliquot is added over the first half of the period, and at least one quarter of the aliquot over the second half of the period; a) mixing the protein carrier and saccharide, and b) adding the aliquot of carbodiimide required to perform the conjugation over a period of 1 minute to 6 hours, wherein at least one quarter of the aliquot is added over the first half of the period, and at least one quarter of the aliquot over the second half of the period; the aliquot of carbodiimide is 0.01 to 3 mg carbodiimide / mg saccharide; the saccharide is present at a final concentration of 0.5-50 mg/ml in step b); the protein carrier is present at a final concentration of 1-50 mg/ml in step b); the initial ratio of protein carrier to saccharide is 4:1 to 1:1 (w/w); the reaction pH in step b) is maintained at pH 4.5-6.5, or pH 4.5-7.5 if a compound is present in step b) which keeps the reaction intermediate stable; the temperature of the reaction in step b) is maintained at 4-37°C.