The present invention relates to a method for preventing the adsorption of lactoferrin to a storage container or a reaction container characterized by regulating the pH value of a lactoferrin-containing solution in the container not to exceed 4, or regulating the salt concentration of the solution, for example, at least 1.0 M in the case of NaCl or at least 0.5 M in the case of KCl; a lactoferrin containing solution which has a pH value of at most 4, a sodium chloride concentration of at least 1.0 M or a potassium chloride concentration of at least 0.5 M; and a kit for assaying lactoferrin which comprises the above-mentioned solution as a constituting reagent.