The IDS 25-Hydroxy Vitamin D RIA kit is a complete assay procedure for the extraction and quantitation of 25-OH D and other hydroxylated metabolites in serum or plasma. Addition of Extraction Reagents 1 and 2 to samples and calibrators, causes precipitation of serum proteins and extraction of 25-OH D. Following centrifugation, portions of the supernatant are incubated with 125I-labelled 25-OH D and a highly-specific sheep antibody to 25-OH D. Separation of antibody-bound tracer from free is achieved by a short incubation with Sac-Cel? (anti-sheep IgG cellulose) followed by centrifugation and decanting. Bound radioactivity is inversely proportional to the concentration of 25-OH D.