The IDS 1,25-Dihydroxy Vitamin D RIA kit is a complete assay system for the purification of 1,25D in patient samples by immunoextraction followed by quantitation by 125I RIA. Patient samples are delipidated and 1,25D extracted from potential cross-reactants by incubation for 3 hours with a highly specific solid phase monoclonal anti-1,25D. The immunoextraction gel is then washed and purified 1,25D eluted directly into glass assay tubes. Reconstituted eluates and calibrators are incubated overnight with a highly specific sheep anti-1,25D. 125I 1,25D is added and incubation continued for 1 hrs. Separation of bound from free is achieved by a short incubation with Sac-Cel? followed by centrifugation, decantation and counting. Bound radioactivity is inversely proportional to the concentration of 1,25D.