Aflatoxin B1 is a persistent public health issue in Ghana. Assessment of AFB1 intervention efficacy is currently dependent on long-term biomarkers. This study was designed to determine whether daily AFM1 biomarker levels could be utilized as an early detection method for intervention efficacy. Participants were treated with a refined calcium montmorillonite clay (UPSN) or a placebo (calcium carbonate) in a crossover study. Urine samples were assessed for AFM1 levels daily. UPSN treatment reduced AFM1 biomarkers by 55% compared to the placebo. This is the first study to show that daily urinary AFM1 levels can be used as a biomarker of internal aflatoxin B1 exposure in short-term intervention trials to determine efficacy. © 2013 Informa UK Ltd. All rights reserved: reproduction in whole or part not permitted.