the sample and antibody the result of the inhibition of such binding of the known amount of conjugates by the unknown sample is compared to the results obtained in the same assay by utilizing known standard solutions of 5 fu in determining the amount of 5 fu in an unknown sample the sample the conjugates formed from the compounds of formula ii b and the antibody may be added in any order various means can be utilized to measure the amount of conjugate formed from the compounds of formulae ii a and ii b bound to the antibody one method is where binding of the conjugates to the antibody causes a decrease in the rate of rotation of a fluorophore conjugate the amount of decrease in the rate of rotation of a fluorophore conjugate in the liquid mixture can be detected by the fluorescent polarization technique such as disclosed in u s pat nos 4 269 511 and 4 420 568 on the other hand the antibody can be coated or absorbed on nanoparticles so that when these particles react with the 5 fu conjugates formed from the compounds of formulae ii a and ii b these nanoparticles form an aggregate however when the antibody coated or absorbed nanoparticles react with the 5 fu in the sample the 5 fu from the sample bound to these nanoparticles does not cause aggregation of the antibody nanoparticles the amount of aggregation.