1) Clearance rate of intravenously administered radioactive vitamin B,, in untreated pernicious anaemia patients is slow in 50 per cent of cases studied.(2) Administration of labelled vitamin B,, bound to slow serum to 'normal clearing' pernicious anaemia subjects or to normal persons is followed by normal clearance rate.(3) Radioactive vitamin B,, bound to normal serum is cleared at a normal rate in pernicious anaemia patients with slow clearance rate. (4) Radioactive vitamin B,, bound to slow serum when mixed with normal serum in proportions of 1:l or 1 : O S is cleared at a normal rate in 'slow clearing' pernicious anaemia patients.( 5 ) These studies suggest the presence of a 'serum factor' in normal serum and some patients with pernicious anaemia in relapse which enhances clearance of vitamin B,, bound to 'slow serum'.

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