Mutants of Escherichia coli (pdx B and pdx C) which are blocked in the biosynthesis of pyridoxol (vitamin B 6 ) showed a growth response to 4-hydroxy-l-threonine. This observation constitutes the first direct evidence in support of the view that 4-hydroxy-l-threonine is implicated in the biosynthesis of vitamin B 6 . 1-Aminopropan-2,3-diol, the decarboxylation product of 4-hydroxy-l-threonine, does not support the growth of these mutants. Deuterium from deuterium-labelled 1-aminopropan-2,3-diol was not incorporated into pyridoxol.