Highly stable cholesterol standard solution comprising a known amount of cholesterol together with 1 to 20 volume percent of a primary or secondary aliphatic alcohol containing up to 4 carbon atoms; 5 to 20 volume percent hydroxy-polyethoxydodecane or 40 to 60 volume percent diethyleneglycol monobutyl ether; and 0.5 to 5 weight percent sodium chloride and/or 0.01 to 0.5 weight percent alkyl dimethyl benzyl ammonium chloride with 8 to 18 carbon atoms in the alkyl radicals.