1,10-phenanthroline-2,9-dicarboxylic acid and novel derivatives thereof such as 4,7-diphenyl-1,10-phenanthroline-2,9-dicarboxylic acid and 4,5,9,14-tetraaza-(1,2,3,4)-dibenzanthracene-3,5-dicarboxylic acid can be coupled to proteins through coupling groups to form conjugates which form higly fluorescent chelates in the presence of lanthanide salts. Derivatives of the above acids reactive toward proteins can be readily manufactured by relatively simple synthetic routes, and are useful in fluorescent immunoassay.