Bone marrow from adult rats fed a vitamin B6 deficient diet for two weeks was found to have reduced numbers of neutrophils, erythroid cells and small lymphocytes. The numbers of transitional cells were not reduced. Since the small lymphocyte population in the bone marrow consists of B cells and null cells in approximately the same proportion, it was concluded that both cell types were reduced as a result of the deficiency. A complete recovery in the cellularity of the bone marrow was brought about by returning the vitamin B6 deficient animals to a normal diet for four weeks. Other effects of the two-week vitamin B6 deficient diet included a failure to gain weight, a decrease in thymus weight and a reduction in the numbers of white blood cells in the peripheral blood. All of these defects were corrected after the animals had been fed a normal diet for four weeks.