SummaryThe activity of pyridoxal phosphate-dependent enzymes was investigated in pyridoxine deficiency. Depression of the activity was most pronounced in serine and homoserine dehydratase, whereas the level of serine hydroxymethylase was not changed. Regeneration of liver after partial hepatectomy in pyridoxine deficiency proceeded at the normal rate as measured by the increase in weight and the amounts of protein, RNA and DNA of the regenerating tissue. The rate of incorporation of thymidine-3H was also unchanged. In pyridoxine deficiency, the incorporation of radioactivity from serine-3-14C into DNA of the regenerating tissue was essentially the same as in the control. This finding together with the resistance of serine hydroxymethylase to the deficiency indicate that the formation of C1 units from serine is not impaired.