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The nucleoside psicofuranine ( Eble et , al ., 1959 ; Yuntsen, 1958) , I, has demonstrated antibacterial efficacy in mice (Lewis, 1959) and antitumor activity in rats upon oral administration (Evans and Gray, 1959) . Significant absorption from the gastrointestinal tract of humans failed to occur, as determined by blood level measurements using the microbiological assay of Hanka (1959) and the chemical method of Forist (1959) . Basicity due to the 6-amino group, and the additional non-lipophillic character ascribed to the four hydroxyl groups are the suspect molecular features which may account for poor absorption (Schanker, 1960) . This communication describes the first chemical modification of a nucleoside which dramatically alters its oral absorption pattern in humans.

The α,e-diaminopimelic acid auxotroph Escherichia coli strain 173-25 has been shown to incorporate γ-methyldiaminopimelic acid into its cell walls. Nutritional studies utilizing this compound have shown that γ-methyldiaminopemelic acid replaces α,e-diaminopimelic acid only when lysine is also supplied in the medium. The thesis is advanced that the only functional pathway of lysine biosynthesis in E. coli is via diaminopimelic acid.