A simplified microbiological assay for determining the folate content of red cells is described. As in previously reported methods Lactobacillus casei is used as test organism but two modifications are introduced. First, haemolysis is carried out in water containing 1 g.% of ascorbic acid; secondly, haemolysates are not incubated before the assay. Using this assay, recovery of pteroylglutamic acid added in two different concentrations to five different whole blood samples was 97·0 ± 1·9 S.E. % and 106·1 ± 4·7 S.E. % respectively. The coefficient of variation of the assay was between 11·2 and 15·0%.