Four analogues of vitamin D3 with an oxygen atom in the side chain skeleton were synthesized to determine whether their differentiation-inducing activity could be separated structurally from their activity to induce hypercalcemia. The order of the in vitro potency to reduce nitroblue tetrazolium in human myeloid leukemia cells (HL-60) was 22-oxa-1 alpha, 25-(OH)2D3 greater than 1 alpha, 25-(OH)2D3 greater than 20-oxa-1 alpha, 25-(OH)2D3 not equal to 22-oxa-1 alpha-(OH)D3 greater than 1 alpha-(OH)D3 greater than 20-oxa-1 alpha-(OH)D3. 22-Oxa-1 alpha, 25-(OH)2D3 was also about 10-times more potent than 1 alpha, 25-(OH)2D3 in suppressing proliferation and inducing differentiation of mouse myelomonocytic leukemia cells (WEHI-3), but the former was much weaker than the latter in inducing the release of 45Ca from prelabeled fetal mouse calvaria. These results suggest that the differentiation-inducing activity of vitamin D compounds can be separated structurally from their activity to induce hypercalcemia.