Although 1α,25(OH) 2 D 3 induces HL-60 cells to differentiate into monocytes, whether or not these monocytes revert to native promyelocytes is not clear. To investigate this question, HL-60 cells were treated with 1α,25(OH) 2 D 3 , and divided into CD14-positive and -negative cells with a cell sorter. These two populations were cultured with or without 1α,25(OH) 2 D 3 . Sorted CD14-positive HL-60 cells, treated with 1α,25(OH) 2 D 3 for 7 days, reverted to CD14-negative cells and promyelocyte-like cells if 1α,25(OH) 2 D 3 was removed from the medium. We conclude that the 1α,25(OH) 2 D 3 -induced differentiation of HL-60 cells into monocytes is reversible, and continuous administration of 1α,25(OH) 2 D 3 is required for the differentiation of HL-60 cells.