The relative contents of CL-19-131I(131I-19-iodocholest-5-en-3β-ol)and NCL-6-131I(131I-6β-iodomethyl-19-norcholest-5(10)-en-3β-ol) in seven batches of 131I-iodocholesterol prepared by Shanghai First Chemical Reagent Factory were determined by means of thin layer chromatography. It was found that when the ratio of CL-19-131I to NCL-6-131I was calculated
there was a negative correlation between this ratio and the adrenal gland uptake of 131I.A negative correlation was also found between this ratio and the heat stability of 131I-iodocholesterol.Therefored NCL- 6- 131I appears to be the better imaging agent for adrenal medulla.