In order to assess the significance of human malignancy-associated nucleolar antigen P24 in the diagnosis of tumors,552 specimens of human tissues were stained and observed with immuno - fluorescent and ABC methods using anti-P24 monoclonal antibody MA2.It was found that P24 was positive in the nucleolus of 97% cases of malignant tumors and in 50% of atypical hyperplastic tissues but negative in 98.6% cases of benign tumors and in all the normal or nontumorous tissues.On the basis of these findings,it is believed that P24 is highly specific to malignant tumors and will be a useful marker for the differential diagnosis between malignant and benign tumors.