1. 1α,25-Dihydroxy-25-hemisuccinate cholecalciferol has been synthesized and conjugated to bovine serum albumin.
2. This conjugate is immunogenic; when injected into rabbits antibodies of high affinity for 1α,25-dihydroxycholecalciferol were obtained.
3. Vitamin D metabolites lacking the 1α-hydroxy group were of lower cross-reactivity with the antibodies.
4. By using these antibodies and 1α,25-[23,24-3H]dihydroxycholecalciferol as tracer a sensitive radioimmunoassay has been developed capable of detecting 20 pg of 1α,25-dihydroxycholecalciferol.