compounds wherein the phenol moiety is labeled with 125 i or 131 i the radiolabeled compounds are useful in radiochemical assaying for folic acid str1 application_number US 94200278 A description compounds of the present invention are prepared according to scheme a str4 these compounds are separated by coventional chromatography techniques the residual acid groups may be esterified by reaction with diazomethane or acid catalyzed esterified with alcohols alternately one of the carboxylic acid groups of the glutamic acid portion of the starting folic acid can be esterified providing a single condensation product esters are converted to acids by conventional base catalyzed hydrolysis radioactive iodine 125 i or 131 i is conveniently introduced into the phenol ring of tyrosine by the chloramine t method of greenwood et al biochem j 89 114 1963 compounds of the present invention are most conveniently used as the acid addition salt of organic and mineral acids such as hydrochloric hydrobromic hydroiodic sulfuric nitric benzoic acetic trichloroacetic toluenesulfonic acids and the like the following examples are intended to illustrate the present invention and not to limit the invention in spirit or scope folic acid 485 mg 1 1 mmole is suspended in water 8 0 ml and stirred magnetically while a solution of l tyrosyl l glutamic acid u03b1 u03b3 dimethyl ester 39 mg 1 17 mmole in pyridine 8 ml is added to this reaction mixture is added 1 ethyl 3 3 dimethylaminopropyl carbodiimide hydrochloride 2 87 mg 1 5 mmole the mixture is stirred at 4 u00b0 c overnight 20 ml of 0 5 sodium bicarbonate solution is added to the reaction mixture and filtered the filtrate is acidified with 1 n hydrochloric acid to.