The present invention provides a process for the immunological determination of creatinine, wherein creatinine is converted into 1-methylhydantoin, the 1-methylhydantoin formed is incubated in an aqueous medium with antibodies which are directed against a conjugate of a first hydantoin derivative of the general formula: ##STR1## in which R.sup.1, R.sup.2, R.sup.3 and R.sup.4, which can be the same or different, are hydrogen atoms, alkyl radicals containing up to 3 carbon atoms or phenyl radicals, with a first hapten carrier substance suitable for antibody formation, reacted with a conjugate of a second hydantoin derivative of general formula (I) with a second hapten carrier substance, one of the components antibody and conjugate being present in the solid phase or in dissolved form and the other component being present in dissolved form, and the inhibition of the binding reaction between the antibodies and the hydantoin conjugate with the second hapten carrier substance is measured.The present invention also provides a reagent for the immunological determination of creatinine, wherein it contains creatinine iminohydrolase, antibodies against a conjugate of a hydantoin of the general formula: ##STR2## in which R.sup.1, R.sup.2, R.sup.3 and R.sup.4, which can be the same or different, are hydrogen atoms, alkyl radicals containing up to 3 carbon atoms or phenyl radicals, with a first hapten carrier substance, a conjugate of a hydantoin of general formula (I) with a second hapten carrier substance which does not cross-react substantially with the first hapten carrier substance and a buffer substance.