There is limited knowledge about which internal and external factors influence respiratory effects of occupational endotoxin exposure. Relationships between endotoxin exposure, work-related respiratory symptoms, and acute peak flow changes were studied in 97 shift workers in the potato processing industry. For each worker, across-shift peak flow changes were determined for morning, afternoon, and night shifts. A higher endotoxin exposure was associated with an increased prevalence of work-related symptoms, a smaller peak expiratory flow (PEF) increase across the morning shift, and a larger PEF decrease across afternoon and night shifts. The largest effects occurred during the afternoon shift. No consistent differences between smokers and nonsmokers, and between atopics and nonatopics were observed. We conclude that endotoxin exposure in the potato processing industry is related to across-shift peak flow changes and the occurrence of work-related respiratory symptoms. The host factors smoking and atopy are not important confounding or effect-modifying factors in these relationships. In respiratory health studies among shift workers, it is important to investigate all work shifts before drawing definitive conclusions about exposure-response relationships.