Recent interest has been revived in the human MN blood group system due to the discovery of the MN precursor substance (T-antigen) on certain malignant, but not benign or normal tissues. Methods for preparing T-activated erythrocytes, anti-T lectin, anti-T absorbed human serum and purified anti-T antibody are described and should be useful not only to the researcher involved in this new line of investigation, but also to the routine blood bank faced with the occasional problem of erythrocyte polyagglitinability.