Current experiments are concerned with: (1) biochemical mechanisms in the Killmann experiment: a critique of the "deoxyuridine suppression test"; (2) development of a sensitive enzymatic assay for picomole quantities to dTMP in tissue extracts; (3) studies on the incorporation of uracil into DNA; (4) studies of one-carbon metabolism of normal and leukemic leukocytes; (5) studies of folate and vitamin B12 uptake in cultured lymphocytes; (6) development of a new assay for folylpolyglutamate synthetase and partial purification of the enzyme from rat liver; (7) binding of human transcobalamins to rat liver cell fractions; and (8) studies of the microbiological (Lactobacillus leichmannii) assay for serum vitamin B12: characterization of serum vitamin B12 analogs that are not detected by the microorganism.