Protein labeling reagents

A method of radiolabeling a protein, comprising reacting a protein with a chelating compound of the formula:

• ##STR28## wherein:
• each X is independently chosen from O, S, NH, or NR.sub.2, X' is independently chosen from S, NH or NR.sub.2, wherein R.sub.2 represents a substituent selected from protecting groups and electron withdrawing groups;
• R.sub.1 represents a protecting group, wherein each R.sub.1 is a separate protecting group or the two R.sub.1 symbols are taken together to represent a single protecting group;
• and
• each T is independently chosen from hydrogen, lower alkyl groups of from 1 to about 6 carbon atoms, electron withdrawing groups, and lower alkyl groups of from 1 to about 6 carbon atoms substituted with electron withdrawing group(s);
• to form a protein-chelating compound conjugate, then reacting the conjugate with a radionuclide metal in chelatable form, thereby producing a radionuclide metal chelate attached to said protein.