There is provided an immunoassay by which the amount of human medullasin present inside granulocytes, which are one leukocyte component in blood, can be determined with high accuracy and with good reproducibility. Also provided is an immunoassay of medullasin, wherein when measuring the medullasin in blood using an anti-human medullasin antibody, the determination of the amount of human medullasin in a blood sample using said anti-human medullasin antibody is carried out after treating the blood sample with an aqueous liquid having a specific osmotic pressure different to the osmotic pressure of blood to completely break up the leukocytes. Also provided is a method of diagnosing multiple sclerosis characterized in that the human medullasin content of a blood sample is measured using an immunoassay, and the onset of multiple sclerosis and the extent of the disease is diagnosed according to the size of, or changes in, this measured value.