Men exposed to high concentrations of bacterial single-cell protein (Pruteen) dust have complained of "sticky eyes" and influenza-like symptoms. Over four years, surveillance of a work force of 70 men with a programme that included a respiratory health questionnaire, skin prick testing, serum testing for precipitating antibodies, lung function measurements, and chest radiography has shown no evidence of allergic reaction or of exposure-related deterioration in lung function. The symptoms experienced may be attributable to exposure to lipopolysaccharide. The results show that Pruteen can be produced and handled without adverse effects on health, provided that adequate measures are taken to prevent exposure to high concentrations of dust.