Under Section 402(a)(3) of the Food, Drug and Cosmetic Act the distribution of foods which may contain repulsive or offensive matter, considered as filth, is prohibited. Filth includes contaminants such as rat, mouse, and other animal hairs and excreta; whole insects, insect parts, and excreta; and other extraneous materials which, because of their repulsiveness, would not knowingly be eaten or used. The presence of such filth renders food adulterated. Some foods, even when produced under good manufacturing practice, contain low levels of natural or unavoidable defects such as dirt, insect parts, and hair that are not hazardous to health. Although the health hazard is eliminated, most consumers find the presence of "any" visible filth contaminant such as hair in a food product objectionable. The objective of this study was to determine when a defect (hair) in a food (mushrooms) is visible to a consumer by testing the visual acuity of a consumer panel under controlled laboratory conditions. Of the panelists participating in the study, when presented with a single hair on a flat surface with a solid-colored background, 27% were able to detect a 1-mm hair, 58% were able to detect a 3-mm hair, and 75% were able to detect a 10-mm hair. Twenty-five percent of the panelists surveyed were able to detect a 5- or 10-mm hair on sliced, canned mushrooms. The experimental design and results of this study are discussed.