Efforts to reduce the incidence of diarrheal infections in which enteropathogens are endemic have focused on education about the importance of hand washing to interrupt transmission of such organisms. Since the effectiveness of health education depends on an understanding of the recipients' ideas and customs, we studied perceptions of cleanliness and the role of soap and hand washing in two poor Bangladeshi communities, one rural and one urban. We found that ideas about cleanliness generally are not based on germ theory; cleanliness is viewed in a larger, socioreligious context of purity vs. impurity. Washing serves both physical and spiritual needs and is performed according to defined patterns that may not effectively interrupt transmission of microorganisms. Soap is regarded as a cosmetic rather than an agent for removal of microorganisms.