U937 is a human-derived lymphoma cell line that has monoblastic properties and high-affinity receptors for 1 alpha,-dihydroxyvitamin D3. Incubation of these cells with the vitamin D metabolite at 10 nM for 5 days produced marked stimulation in adherence and ingestion of Staphylococcus aureus (645% of control) and of C3b receptor (CR1) expression (292% of control) and a slight increase in hexose monophosphate shunt activity without changing cell growth rates or Fc fragment receptor expression. The changes in cellular association of S. aureus and the CR1 were detected as early as 48 hr of incubation and peaked between 3 and 5 days. Similar changes in the CR1 were induced by 25-hydroxy- and 24,25-dihydroxyvitamin D3 at micromolar concentrations. Dexamethasone, hydrocortisone, and progesterone had no effect on CR1 expression. U937 cells incubated in the presence of vitamin D metabolites exhibited a change in their phenotype. These results suggest that vitamin D metabolites may contribute to monocyte/macrophage differentiation.