Employees at six sewage treatment plants and three drinking water plants were interviewed for the presence of specific medical symptoms. Serum immunoglobulin concentrations, white blood cell counts and fibrinogen degradation product concentrations (FDP) in urine were determined as were the number and species of airborne Gram negative rods in order to characterise exposure to aerosols of sewage water. The highest number of bacteria was found in areas where the sewage water was agitated. A significantly higher proportion of employees at sewage treatment plants reported skin disorders, diarrhoea, and other gastrointestinal symptoms than the control group. No significant differences were found between the groups for white blood cell count or serum immunoglobulin concentrations, except that IgM concentrations were slightly higher in the sewage workers. Some workers had serum transaminase concentrations in excess of normal; some of these returned to normal after the summer holiday. Among non-smokers a higher proportion of sewage treatment workers had increased amounts of FDP in urine. It is conceivable that the symptoms observed were caused by toxins from Gram negative bacteria.