The physiologically active form of vitamin D, 1,25 dihydroxyvitamin D3 [1,25(OH)2D3], was found to inhibit erythroid differentiation of human leukemic K562 cells. Differentiation was induced by 1 mumol/L arabinocytosine (Ara-C), 40 mumol/L tiazofurin, 1 mumol/L aphidicolin, or 1 mumol/L hydroxyurea, and was monitored daily by the appearance of hemoglobin in an increasing proportion of cells. Pretreatment for 48 hours with 2.4 x 10(-8) mol/L 1,25(OH)2D3, a concentration that is also optimal for induction of monocytic differentiation of HL-60 cells, reproducibly inhibited subsequent induction of erythroid differentiation by all of the above inducers, and modified the morphologic changes that Ara-C produced in these cells. The inhibition of hemoglobinization was approximately 50% irrespective of the degree of differentiation produced by the various inducers, but growth inhibition associated with exposure to the inducers was not affected by 1,25(OH)2D3. Similar inhibition of differentiation by 1,25(OH)2D3 was observed in mouse erythroleukemia cells MEL-D1B treated with 5 mmol/L hexamethylenebisacetamide. The inhibitory effect of 1,25(OH)2D3 on erythroid differentiation of K562 cells was abrogated by cyclohexamide (20 micrograms/mL), an inhibitor of protein synthesis. The mRNA for 1,25(OH)2D3 receptor (VDR) was detected in K562 cells, and was downregulated by a 96-hour exposure to 1,25(OH)2D3 or a 48-hour exposure to Ara-C. The presence of VDR mRNA suggests a physiologic role for 1,25(OH)2D3 in K562 cells that are precursors of erythroid cells. This role is perhaps to shift the pathways of differentiation from the erythroid to the monocytic lineage.