Measurement of red cell folate with 75Se-selenofolate radioassay

75Se-selenofolate radioassay was adapted for the purpose of red cell folate measurement. The results of assay of 124 whole blood samples by Lactobacillus casei microbiological assay and by radioassay using the adapted selenofolate method were concordant (r = 0.8). Red cell folate levels by the radioassay were below 170 microgram/1 in people who had haematological features of folate depletion. The distribution of results among reputedly healthy subjects extended down to 140 microgram/1.