Counts of airborne bacteria colony forming particles (BCFP) and fungi were made at intervals throughout one year in three calf houses using a six stage Andersen Sampler. House 1 was insulated, mechanically ventilated and heated, House 2 was insulated and provided with a controlled natural ventilation system, while House 3 was uninsulated with natural ventilation. Each house contained 36 bought-in bull calves. Every six weeks the 12 oldest calves were removed and replaced by 12 four-week-old calves. The mean count of BCFP was highest in House 2 (101.6 X 10(3) m-3) and lower in House 3 (67.6 X 10(3) m-3). The mean count of aerial fungi was significantly lower in House 3 (40.5 X 10(3) m-3) than in Houses 1 and 2 (119.3 X 10(3) m-3 and 127.1 X 10(3) m-3, respectively). The count of aerial BCFP and fungi showed large seasonal fluctuations, but there was a general trend towards lower counts during the winter period. The mean incidence rate of respiratory disease among the experimental calves was 67.7%. The highest mean incidence rate was recorded in House 2, but differences between houses were not significant. The results are discussed in relation to the environmental requirements for raising of calves, and in the light of the current concept of air hygiene as a major predisposing factor in the web of causation of calf respiratory disease.