The synthesis of the 20-epi-23-oxa-24a,24b-dihomo analogue (MC 1357) of 1alpha,25-dihydroxy-vitamin D3 is described. In classical receptor binding and in vivo calcemic effect assays MC 1357 is slightly less potent than the natural hormone. However, in its in vitro effects on cancer cell differentiation and proliferation and on the mixed lymphocyte reaction it is 2 to 3 orders of magnitude more potent.