144 workers of two Ukrainian fodder production plants have been examined. Respiratory troubles such as chest tightness, cough with phlegm, wheezing were revealed not only in the group of workers exposed to high levels of endotoxins but also in the group exposed to low concentrations. The prevalence of ODTS (Organic Dust Toxic Syndrome), mucosal irritation syndrome, bronchial hyperresponsiveness and obstructive changes of lung function were related to levels of endotoxins in the air of work zone. Respiratory challenge test is the principal to reveal the early signs of obstructive changes of lung function. The protective measures for prophylaxy of the development of occupational diseases in workers exposed to organic dust were proposed.