A colloid solution that exhibits storage stability and pH stability, being useful as viral substitute particles for use in an integrity test for virus removal membrane. In particular, a metal colloid solution characterized in that it comprises metal particles or metal compound particles of 1 to 100 nm average diameter, a water soluble high-molecular-weight dispersant having an N group and water and/or a water soluble organic solvent, the metal colloid solution being stable for a prolonged period of time and being stable in at least pH values ranging from 4 to 11.
