A method of testing for the existence of mastitis in milk from cows either from an individual cow or from a herd of cows, by (a) obtaining a sample of milk to be tested, (b) determining whether immunoglobulin G2 (IgG2) is present in said sample at a level above or below a predetermined level which is typically about 0.05 mg IgG2 per millilitre of milk, by an ELISA comparison with immunoglobulin standards, and (c) classifying the individual cow or herd of cows as mastitis affected if said immunoglubulin is present in said sample at or above said predetermined level.