1. DNA synthesis in the bone marrow and lymphocytes of the vitamin B12-deficient fruit bat (Rosettus Aegyptiacus) has been studied via the deoxyuridine (dU) suppression test.2. In contrast to vitamin B12deficiency in man, the dU suppression test was not abnormal in the vitamin B12-deficient bat, and this correlates with the reported finding that the latter does not develop megaloblastic anaemia.3. The protection of haemopeiesis from vitamin B12deficiency in the bat is probably related to the presence in the bat of separate pools of methylfolate and tetrahydrofolate, which enables the bat to overcome the trapping of methylfolate attendant on vitamin B12deficiency.4. These results confirm the central role of the vitamin B12and folate-dependent pathway ofde novosynthesis of DNA in the pathogenesis of vitamin B12-deficient megaloblastic anaemia.