The biosynthesis of formic acid in the poison glands of formicine ants is closely related to the C-1 metabolism of the glandular cells. Experiments utilizing radiolabeled amino acids revealed that serine is a major precursor, contributing both its α and β carbons to formic acids. 5,10[14C]methylene H4folate and 5,10[14C]methenyl H4folate also serve as precursors of formic acid in the poison gland, suggesting that they are intermediates in the pathway. Furthermore, these H4folate derivatives were isolated from poison glands following incubation with [3-14C]serine and proved radioactive. The glandular cells are also exceptionally rich in the enzymes responsible for these reactions, supporting the proposed pathway. Although this pathway has been established in various organism, the uniqueness of the poison gland system is that it accumulates formic acid to large extent, yet avoids its cytotoxicity. This is made possible by a combination of the biochemical characteristics of the pathway and the special morphological features of the poison gland.