Analyses were conducted on three hay samples taken from a barn in which both the farmer and the dairy cows demonstrated symptoms associated with dust from use of a bedding chopper. Hay extracts were examined for bacteria and fungi and respirable dust clouds were generated from the samples to study airborne contaminants. Sample 1 consisted of hay and hay chaff which was the accumulation of about 5 years between the floor of the mow and the ceiling of the milking floor of the barn. Sample 2 was newly purchased from a local source. Sample 3 was 1 year old and probably had gotten wet during storage. Total bacteria in samples 1, 2, and 3 were 1.2x10(4), 1.7x10(3), and 2.3x10(5) colony forming units/milligram of dust (CFU/mg), respectively. Gram negative bacteria were determined in Sample 3 to be 4.5x10(6)CFU/mg. Mesophilic fungi in order of Samples 1, 2, and 3 were determined to be present at 1.2x10(4), 8.5x10(4), and 1.1x10(6)CFU/mg. Xerophilic fungi correspondingly were 1.0x10(4), 7.2x10(4), and 1.1x10(6)CFU/mg. Thermotolerant fungi were present at 2.9x10(2), 9.2x10(1), and 8.09x10(2)CFU/mg, respectively. Sera from eight of ten sick cows and two healthy cows from the same barn and six cows from other farms produced precipitin bands on counter immunoelectrophoretic analysis with Sample 3. The authors conclude that dusts generated from hay samples associated with an outbreak of respiratory illness in cows contained high levels of fungi and bacteria.