We developed a polarization fluoroimmunoassay for the specific detection of methamphetamine in urine. Sheep antiserum raised against methamphetamine was premixed with a fluorescein-labeled amphetamine tracer in a single reagent. The use of labeled amphetamine gave a more sensitive and more specific assay than did the use of a labeled methamphetamine tracer. The assay system had equal sensitivity for the d- and l-isomers of methamphetamine while the cross reactivity of amphetamine and other structurally related compounds was 3% or less. With a positive/negative cutoff concentration set at 1 mg/L, the polarization fluoroimmunoassay reliably identified human urines that had been reported positive for methamphetamine by an established gas-liquid chromatographic method.