1. Eleven thousand two hundred and twenty-eight sera from persons, mostly blood donors, living in Kent and seven counties in East Anglia, have been tested and 379 (3·4%) found to have complement-fixing antibody to Rickettsia burneti at a serum titre of 1/10 or greater.
2. A group of 396 donors in Kent and East Anglia, composed of 190 donors with antibody, and 206 donors from those without antibody to serve as controls, were interviewed, and an epidemiological history was obtained on the possible ways in which they had been exposed to R. burneti during the period 1942–53.
3. A comparison of the epidemiological histories of the positive and negative donors showed that donors in Kent who had constant occupational exposure to cattle and sheep including unprocessed materials from them, or who had had a raw milk supply to their homes for all or part of the period 1942–53, more frequently had antibody than those donors without such qualities. In East Anglia the positive serological state among donors was associated only with the use of raw milk and not with occupational exposure to animals. There were suggestive findings among Kentish donors that those who visited farms or other potentially infectious places, either during the course of their job or during their leisure, also experienced greater risk of infection.
4. In general, these findings of infection from occupational exposure, from visits to infectious localities, or from the use of raw milk in the home, confirmed the tentative epidemiological conclusions drawn from a previous investigation of patients with Q fever. It was not found in this survey, however, that residence near dairy farms, receipt of objects from potentially infectious localities, or contact with persons from such localities was associated with a special risk of infection.
5. No association was found between the donors' serological state and the keeping of dogs, cats, poultry, pigs, rabbits, psittacine birds in the vicinity of the home or its infestation by mice, rats or mosquitoes.