The objective of this cohort study was the evaluation of the efficacy of an immune modulator in the prophylaxis, metaphylaxis and treatment of enzootic bronchopneumonia (EBP). A total of 552 dairy calves on 13 farms were included. Groups of calves were included in the study if the prevalence of respiratory disease on the first visit was at least 50%, and the number of animals aged 2-16 weeks was at least 10. Controls were treated with a placebo. Animals of both groups were treated antibiotically if body temperature exceeded 39.5 degrees C. A clinimetric system was devised and used for classifying the animals into one of seven categories (healthy, beginning respiratory disease, acute BP, subacute BP, chronic BP, acute flare-up of chronic BP, and other disease). Animals were examined clinically on days 1, 3, 10, and 28. In animals treated prophylactically, the incidence of BP on day 10 was significantly higher (51%) than in those that received the placebo (31%). In animals treated metaphylactically, the average number of treatment days was significantly lower (2.86) than in the placebo group (3.81). In animals treated therapeutically, the incidence of EBP on day 10 was significantly lower than in the placebo group.