In a leading article on mental symptoms in vitamin B12deficiency (Lancet, 1965), it was pointed out that “though mental illness due to vitamin B12deficiency is uncommon, it is readily treated, and, as in general paralysis of the insane, its protean manifestations and uncertain physical signs call for an easily applied screening test comparable to the Wassermann reaction”. In the correspondence which followed it was suggested that in psychiatric practice today mental symptoms were likely to be found more frequently in association with vitamin B12deficiency than with a positive serological test for syphilis (owing in part to the increasing number of patients with deficiency syndromes following gastro-intestinal surgery); and that estimation of serum B12concentration should be performed much more readily without waiting for clear-cut haematological evidence or neurological complications of megaloblastic anaemia (Hunter and Matthews, 1965). This was emphasized by the evidence of Strachan and Henderson (1965) who described three patients with psychiatric syndromes attributed to vitamin B12deficiency in whom not only peripheral blood but also bone marrow was normal; and the report from a mental hospital in Norway by Edwin et al. (1965) that of 396 patients over the age of thirty years admitted over a twelve months period, 23 (5·8 per cent.) had serum B12concentrations below the critical level of 100 μμg. per ml. as estimated by Euglena gracilis assay. This finding was challenged by Herbert et al. (1965), who suggested that drugs of the phenothiazine group administered to the patients might have suppressed the growth of the organism, but this was disputed by Edwin et al. (1966).