Thirty-five Holstein cows were utilized in a randomized complete block design to evaluate the efficacy of an aluminosilicate clay (adsorbent, FloMatrix, PMI Nutritional Additives, Arden Hills, MN) at reducing aflatoxin M₁ transfer into the milk. Cows were stratified by parity, stage of lactation, and previous milk production. Cows were randomly assigned to 1 of 5 dietary treatments, and treatment was administered for 13 d (n = 7): (1) control (CON), basal TMR with no AF or clay; (2) clay control (4C), basal TMR plus 4 oz clay; (3) AF control (AFC), basal TMR plus 113 ppb AF; (4) AF diet with smaller clay dose (4C+AF), basal TMR plus 4 oz clay and 113 ppb AF; (5) AF diet with greater clay dose (8C+AF) basal TMR plus 8 oz clay and 113 ppb AF. Data were analyzed using the GLM procedure of SAS. Treatment and day were considered independent variables, and milk yield, DM and nutrient intakes, aflatoxin M₁ variables, milk composition, body weight and condition, locomotion, and respiratory rate were dependent variables. A covariate adjustment was used to analyze milk yield and composition from previous milk yield records and DHIA records taken 3 d before the start of the treatment period. Means were separated using Fisher’s Least Significant Difference, and significance was declared when P ≤ 0.05. Tendencies were discussed when 0.05 < P ≤ 0.10. Data were presented as mean ± the largest standard error. Milk yield averaged 29.63, 31.85, 29.95, 29.93, and 31.30 ± 1.762 kg/d for CON, 4C, AFC, 4C+AF, and 8C+AF diets, respectively, and was greatest in cows consuming 4C and 8C+AF diets (P = 0.013). Respiratory rate and BCS were similar across treatments, and 8C+AF cows tended to have a greater locomotion score compared with other treatments (1.71 v. 1.23, P = 0.057). Aflatoxin M₁ concentration, secretion, and transfer were reduced by the addition of clay (P < 0.001), and a dose response was observed. Aflatoxin M₁ concentration averaged < 0.01, 0.00, 1.64, 1.26, and 0.90 ± 0.383 ppb for CON, 4C, AFC, 4C+AF, and 8C+AF cows, respectively. A reduction of transfer of 21.88 and 40.63% was observed for 4C+AF and 8C+AF diets, respectively.