An epidemiological investigation was made of eight employees working in a defined section of an office building who complained of febrile reactions accompanied by respiratory tract symptoms and fatigue. Culture of the water from the humidifier for their section yielded strains of Pseudomonas acidovorans and an unidentified Pseudomonas species. In vitro studies showed that these strains activated the alternative pathway of the complement system in Mg-EGTA supplemented serum, as evidenced by properdin depletion and conversion of C3-proactivator (Factor B) and C3. Whereas the employees with symptoms had significantly reduced properdin values during the symptomatic phase, a slight C3 conversion and antibodies to Pseudomonas spp., staff working in another part of the building with a different ventilation system did not. It was concluded that staff with humidifier fever symptoms had been exposed to Pseudomonas spp. through the faulty ventilation system, and their symptoms were most likely a result of complement activation.