SummaryA group of nineteen normally healthy subjects in a microprocessor factory reported various degrees of work‐related respiratory and systemic symptoms that in some were suggestive of humidifier fever (HF). They all had normal serology against a variety of infectious agents, but serum‐precipitating antibody against antigens in the factory humidifier water was present in twelve subjects, primarily those with symptoms most in keeping with HF. There was extensive serological identity between the antigens from this and other confirmed sources of HF. The antibody‐positive subjects were all nonsmokers and had significantly raised serum IgG. Nine months later, following modification of the humidifier system, the symptoms had resolved in fourteen of the nineteen subjects, the precipitins were reduced in seven but persisted in five subjects, and the total IgG levels were significantly reduced in all subjects. The measurement of total, as well as specific, IgG may be of value in assessing this di