A metabolite of labetalol that is responsible for previous reports of false-positive assays for amphetamines by thin-layer chromatography and immunoassay has been identified. The compound, 3-amino-1-phenylbutane (APB), an oxidative metabolite of labetalol, was initially identified in a patient's urine by gas chromatography-mass spectrometry and confirmed by analysis of the pure material. Gas chromatography showed a single major peak with relative retention time indistinguishable from that of methamphetamine, both underivatized and as the pentafluoropropionyl derivative. By classical thin-layer chromatography, APB migrated identically to methamphetamine but showed ninhydrin color development characteristic of amphetamine. APB was shown to cross-react approximately 2% with the Abbott TDx amphetamine/methamphetamine II kit, 10% with the Syva EMIT d.a.u. polyclonal amphetamine class kit, and 3% with the Syva EMIT d.a.u. monoclonal amphetamine kit. This degree of cross-reactivity is sufficient to cause false-positive immunoassays when cutoffs of 300 ng/mL are used.