PURPOSE: To achieve an assay of creatinine at a high accuracy and with a high sensitivity, by using a reagent composition comprising picric acid or dinitrobenzoate, a basic compound and an agar.

CONSTITUTION: A reagent compound containing 0.03W0.20g/dl of picric acid or 3,5 dinitrobenzoate, 0.04W4 N of basic compound and 1W5wt% of agar as effective components is used for the assay of creatinine. When a trace of sample is spotted on the surface of the gelatinous reagent 2 thus arranged, a coloring agent (picric acid or dinitrobenzoate) contained in the gel is bonded to creatinine in the sample to discolor and the degree of coloring is reflected accurately according to the concentration of creatinine in the sample. As the gel thus colored is so stable as not to change in the degree of coloring with time thereby enabling the assay of creatinine at a high accuracy with a high sensitivity.