The diagnostic value of the Becton Dickinson Radioassay Kit ( 125I) for the assay of red cell folate has been investigated. The assay was acceptable with regards to precision but was non-linear with changing packed cell volume. Sensitivity of the assay was satisfactory, with 24 of 25 folate deficient patients giving red cell folate values which fell below the reference range. Specificity of the assay in the detection of folate deficiency was less satisfactory. As with microbiological assays, a considerable proportion of vitamin B12 deficient patients had low red cell folate values. In addition, low concentrations were found in 12% of patients who were unlikely to be deficient in either vitamin B12 or folic acid.