A separation fluoroimmunoassay system for phenytoin was established based on the use of a specific rabbit antiserum, a fluorescein-labelled ligand, and precipitation of the antibody-bound fraction of the labelled ligand with sodium sulphate. Simple measures were taken to obviate non-specific binding and matrix effects. Either the free fraction (in the supernatant) or thebound fraction of the labelled ligand was quantitated fluorimetrically. Assays of patient serum samples by either method correlated well with established gas-liquid chromatographic and radioimmunoassay techniques. Advantages of a separation based procedure as compared with previously described non-separation hapten fluoroimmunoassay techniques are that only simple instrumentation and assay reagents are required, and that the separation step may enable the removal of any interfering intrinsic fluorescence of serum samples.