A kit for fractionation of small dense LDL cholesterol (sdLDL-C) in a sample, comprising: (i) a first reagent composition containing a surfactant that acts on lipoproteins other than the small dense LDL (sdLDL) and having the enzymes cholesterol esterase, cholesterol oxidase, and sphingomyelinase; and (ii) a second reagent composition for quantifying the sdLDL-C, wherein the second reagent composition comprises a surfactant that acts on the sdLDL-C; wherein in an absorption spectrum after storing the first reagent composition at 37°C for 2 weeks, when an absorbance at a wavelength of 400 nm is ABS400, and an absorbance at a wavelength of 450 nm is ABS450, a ratio R1 represented by ABS400/ABS450 is 0.90 or more and 3.00 or less, and in an absorption spectrum after storing the second reagent composition at 37°C for 2 weeks, when an absorbance at a wavelength of 400 nm is ABS400 and an absorbance at a wavelength of 450 nm is ABS450, a ratio R1 represented by ABS400/ABS450 is 0.90 or more and 8.00 or less, and wherein the first reagent composition satisfies one or two conditions of Conditions 1 to 3 and the second reagent composition does not satisfy the one or two of Conditions 1 to 3 and satisfies all other conditions, wherein the Conditions 1 to 3 are: Condition 1: containing a coupler; Condition 2: containing an iron complex; and Condition 3: having peroxidase. (i) a first reagent composition containing a surfactant that acts on lipoproteins other than the small dense LDL (sdLDL) and having the enzymes cholesterol esterase, cholesterol oxidase, and sphingomyelinase; and (ii) a second reagent composition for quantifying the sdLDL-C, wherein the second reagent composition comprises a surfactant that acts on the sdLDL-C; wherein in an absorption spectrum after storing the first reagent composition at 37°C for 2 weeks, when an absorbance at a wavelength of 400 nm is ABS400, and an absorbance at a wavelength of 450 nm is ABS450, a ratio R1 represented by ABS400/ABS450 is 0.90 or more and 3.00 or less, and in an absorption spectrum after storing the second reagent composition at 37°C for 2 weeks, when an absorbance at a wavelength of 400 nm is ABS400 and an absorbance at a wavelength of 450 nm is ABS450, a ratio R1 represented by ABS400/ABS450 is 0.90 or more and 8.00 or less, and wherein the first reagent composition satisfies one or two conditions of Conditions 1 to 3 and the second reagent composition does not satisfy the one or two of Conditions 1 to 3 and satisfies all other conditions, wherein the Conditions 1 to 3 are: Condition 1: containing a coupler; Condition 2: containing an iron complex; and Condition 3: having peroxidase. wherein in an absorption spectrum after storing the first reagent composition at 37°C for 2 weeks, when an absorbance at a wavelength of 400 nm is ABS400, and an absorbance at a wavelength of 450 nm is ABS450, a ratio R1 represented by ABS400/ABS450 is 0.90 or more and 3.00 or less, and in an absorption spectrum after storing the second reagent composition at 37°C for 2 weeks, when an absorbance at a wavelength of 400 nm is ABS400 and an absorbance at a wavelength of 450 nm is ABS450, a ratio R1 represented by ABS400/ABS450 is 0.90 or more and 8.00 or less, and wherein the first reagent composition satisfies one or two conditions of Conditions 1 to 3 and the second reagent composition does not satisfy the one or two of Conditions 1 to 3 and satisfies all other conditions, wherein the Conditions 1 to 3 are: Condition 1: containing a coupler; Condition 2: containing an iron complex; and Condition 3: having peroxidase. Condition 1: containing a coupler; Condition 2: containing an iron complex; and Condition 3: having peroxidase.