An investigation of the factors that influence the production of dense aerosols of Micropolyspora faeni and Thermoactinomyces vulgaris from hay cultures revealed that the density, freshness, and moisture content of the hay were important influences. Dry aerosols were produced under optimum conditions from hay cultures of either actinomycete and from sterile hay for inhalation challenges to unimmunized rabbits. Depressions in arterial oxygen tensions and hemolytic complement were monitored after such challenge. This investigation showed that hay cultures are capable of inciting a hypersensitivity-like reaction in the lungs, even in the absence of immunization to the organisms contained within the hay, but the entire hypersensitivity reaction cannot be attributed to M. faeni and T. vulgaris alone. Hay itself or other microbial or chemical components appear to have some heat-stable component that possibly contributes to the pathogenesis of the disease.