Vitamin D3 and its hydroxylated metabolites are normally in thermal equilibrium with their previtamin D isomers. To evaluate the biologic activity of 1α,25-dihydroxyprevitamin D3, we synthesized 19-nor analogs of 1α,25-dihydroxy(pre)vitamin D3 because the absence of a C19 methylene group prevents the isomerization of these analogs. The affinity of 1α,25-(OH)2D3-19-nor-D3 for the intestinal vitamin D receptor and plasma vitamin D binding protein was mildly decreased [30 and 20% of the affinity of 1α,25-(OH)2D3, respectively], but the affinity of 1α,25-(OH)2-19-nor-previtamin D3 was only 1 and 6% of that of 1α,25-(OH)2D3 for the receptor and DBP, respectively. The in vitro effects on human promyeloid leukemia (HL-60 cell) differentiation and osteocalcin secretion by human osteosarcoma (MG-63) cells by 1α,25-(OH)2-19-nor-D3 were nearly identical to those of 1α-25-(OH)2D3, whereas 19-nor-previtamin D3 showed poor activity (2%). The in vivo calcemic effects of both analogs, studied in vitamin D-deficient chicks treated for 10 consecutive days with the analogs, showed no activity of the previtamin D3 analog and reduced calcemic effects (≤ 10%) of 1α,25-(OH)2-19-nor-D3. We conclude that the previtamin D form of 1α,25-(OH)2D3 has lost most of its biologic activity in vitro and in vivo.