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Estimates of the collagen concentration in human ventricles have been made from measurements of the hydroxyproline concentration. In the normal heart the concentration of collagen was higher in the right ventricle than in the left ventricle. Age had no effect on the ventricular concentration of collagen. Hypertrophy in the absence of a valvar lesion was not associated with an increased concentration of collagen but, owing to the increased size of the ventricle, there was an increase in the estimated total mass of ventricular collagen. The concentration of collagen in the left ventricle of patients with aortic stenosis was higher than normal. Ventricular hypertrophy seems to be accompanied by an increase in the total mass of collagen whatever the cause. Whether the concentration changes or not depends on the proportion in which the mass of collagen increases relative to the mass of muscle cells.

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The preparation of high specific activity insulin-131I in a relatively pure state is essential for the immuno-assay of plasma insulin (Berson & Yalow, 1960). The micro\x=req-\ diffusion technique (Banerjee & Ekins, 1961) has provided a safe method for preparing insulin-131I with relatively little chemical damage but is limited in iodination efficiency to a theoretical maximum of 50%. Hunter & Greenwood (1962) have prepared human growth hormone-131I, using chloramine T as the oxidizing agent, with an iodination efficiency over 60%.One of the main difficulties in preparing high specific activity insulin-131I by such methods has been the associated chemical damage of the insulin due to direct contact of the hormone with the oxidizing agent. This would be overcome if a suitable purification technique could be found. A new method of purification of the insulin\x=req-\ 131I has been developed in which the damaged iodinated insulin is removed by buffer elution on a suitable grade dextran column. In this way, using chloramine oxi¬ dizing system and dextran column purification technique, it has been possible to prepare insulin-131I with a specific activity of 1 c/mg., the final product containing a damaged fraction of < 3 %.Freshly dissolved crystalline beef insulin (24-5 u./mg., Burroughs Wellcome Ltd) 2-5 µg. in 0-025 ml. borate buffer, pH 8-0,4-5 mc (0-05 ml.) carrier free, and thiosulphate free 131I in sodium hydroxide (Radiochemical Centre, Amersham) were mixed with 50 µg. chloramine (British Drug Houses Ltd) in 0-025 ml. borate buffer for 1 min. The reaction was terminated by adding 120/xg. sodium metabisulphite in 0-1 ml. buffer. The reaction was carried out in a 10 ml. stoppered glass tube in a glove box. The free 131I was removed by dialysing the iodinated-insulin mixture against distilled water for 2 hr. at 4°c. 1 ml. Fresh human serum was incubated with 0-5 % iodoacetamide for 30 min. at room temperature. The treated serum was stained with 0-01 % bromphenol blue indicator in barbitone buffer pH 8-5 (v:v/5:l) and was then dialysed against 11. barbitone buffer for 2 hr. to remove any excess dye and iodoacetamide. 0-1-0-2 ml. of the stained serum was added to 0-3-0-4 ml. of the iodinated insulin solution as the binding protein for the damaged iodo-compounds. The sample was passed through a 1 g. Sephadex G-75 medium grade (Pharmacia, Uppsala, Sweden)