Equilibrium dialysis was used to determine whether aflatoxin M1 (AFM1) binds to casein. Simulated milk ultrafiltrate (SMUF) containing 10 or 20 ng/ml of AFM1 was dialyzed against SMUF containing casein micelles. After 24 h at 7 degrees C, the casein suspension contained 2.5- or 2.9-fold, respectively, more toxin than that found in SMUF. An average of 17.9 or 55.3 micrograms of AFM1, respectively, per gram of casein was bound. In a separate experiment, milk naturally contaminated with AFM1 was treated with a proteolytic enzyme. An average of 30.7% more toxin was found in treated than in untreated milk. This result also suggests binding of AFM1 by milk protein.