A method for conjugating an isolated antibody to a label or derivatisation reagent, which method comprises contacting the antibody with an activated label or activated derivatisation reagent, or contacting the antibody with the label or derivatisation reagent in the presence of an antibody conjugation reagent, in a buffer system which comprises a monocarboxylic acid buffer compound other than glycine, bearing an amine substituent at the alpha or beta position.