Oral supplementation of clay to dairy cattle has been reported to reduce toxicity of aflatoxin in contaminated feed. The objective of this study was to determine the effects of 3 levels of dietary clay (EcoMix) supplementation after an aflatoxin challenge. Ten multiparous rumen-cannulated Holstein cows (BW = 669 ± 20kg, 146 ± 69 DIM), were assigned to 1 of 5 treatments in a completely randomized replicated 5 × 5 Latin square design. Periods consisted of a 14-d (d 1–14) adaptation period followed by a 3-d aflatoxin challenge (d 14–16; 100 μg of AFB1/kg of diet DM administered intra-ruminally). Cows were sampled daily during the last week of the period (d 14–21). Treatments were: POS, no EcoMix plus aflatoxin challenge; 0.5%, 1%, 2%, percentage of dietary DMI as EcoMix; and CON, no EcoMix and no aflatoxin challenge. Statistical analysis was performed using the MIXED procedure of SAS. Two contrasts CONT1 (POS vs. CON), CONT2 (POS vs. average of 0.5%, 1%, and 2%) were compared along with the linear and quadratic treatment effects. Milk samples were collected daily and tested with AFM₁ snap test (SNP; IDEXX, Inc.). Cows in CON had no positive SNP tests, while cows in POS had 14.2 ± 0.44 (P < 0.001, CONT2) positive SNP tests. Cows supplemented with EcoMix had lower number of positive SNP tests (0.5% = 13.9, 1% = 13.1, 2% = 12.4, SEM = 0.4, P = 0.04) than cows in POS (14.2, CONT2). Cows supplemented with EcoMix through the challenge had lower 3.5% FCM (0.5% = 38.2 kg, 1% = 39.3 kg, 2% = 38.4kg, SEM = 1.8, P = 0.06), ECM (0.5% = 37.1 kg, 1% = 37.1 kg, 2% = 37.1 kg, SEM = 1.6, P = 0.05, and Fat kg/d (0.5% = 1.36 kg, 1% = 1.43 kg, 2% = 1.40 kg, SEM = 0.09, P = 0.09) than cows in POS (41.3 kg, 39.6 kg, 1.5 kg, respectively, CONT2). Serum cholesterol and creatine phosphokinase concentrations were higher for cows fed EcoMix (232.6 mg/dL ± 8.3, P = 0.08, 140.1 U/L ± 9.3, P = 0.04, respectively) than cows in POS (227.4 mg/dL ± 8.4, 126.3 U/L ± 9.3, respectively). In conclusion, oral supplementation of EcoMix decreased the total number of positive SNP tests after an aflatoxin challenge and seems to work as a flow agent for aflatoxin contamination in dairy cattle.