Aflatoxin B1 was detected, and its identity confirmed, in hypoallergenic milk substitutes composed, among other things, of the following ingredients susceptible to possible aflatoxin contamination: soya protein isolate, soy and coconut oils, cornstarch, and corn syrup. Except for one determination all findings were under 1 ng of aflatoxin/ml of formula at drinking concentration. Manufacturers' reserve samples of soya protein isolate, the ingredient thought to be the most likely source of the aflatoxin, were found to be uncontaminated. Reserve samples of other ingredients were not examined. Because hypoallergenic formula may be the major nutrient of an individual with a metabolic defect at infancy, a vulnerable stage of life, available control measures should be used to avoid aflatoxin-contaminated ingredients.