A group of dairy farmers studied 6 years earlier in a field survey was re-surveyed for respiratory symptoms, occupational capability and the presence of antibodies against environmental micro-organisms. Specific IgG antibodies to Micropolyspora faeni, Thermoactinomyces vulgaris, Aspergillus umbrosus and Aspergillus fumigatus were measured by ELISA from the serum samples obtained in the two surveys. Antibody titres remained constant in 70% of both farmers and controls, but where changes took place, the titres against the actinomycete antigens tended to rise, whereas both increases and decreases were detected equally against the Aspergillus antigens. The titre of specific antibody to any of the four micro-organisms, when measured from a single serum specimen, seemed to be of little diagnostic value. Observed changes however, were more diagnostic, in that a fall in titre, especially against the Aspergillus antigens, was closely associated with a definite decrease in exposure, such as after retirement. Increased titres occurred in farmers with continued exposure, and those against the actinomycetes were associated with the appearance of symptoms in previously symptom-free individuals. In a case of farmer's lung which developed in this population during the follow-up period, significant increases were detectable against T. vulgaris and M. faeni.