A new radioimmunoassay, using ³H-cyclazocine, rabbit antiserum and charcoaldexttan separation of bound and free cyclazocine, for the direct analysis of serum cyclazocine is described. This method, which is specific for cyclazocine and has a detection limit of ca. 25 pg/assay tube, was successful in determining the cyclazocine level in the sera of beagles intramuscularly injected with 3 or 10μg/kg cyclazocine. The drug half-life was 90 min ; the apparent distribution volumes were 4.0 and 5.26 liter/kg, respectively. One of the antisera raised in rabbits immunized with dl-cyclazocine derivative-BSA conjugates was highly specific for l-cyclazocine.