Objective: to compare the field efficacy of florfenicol with tilmicosin in the treatment of naturally occurring enzootic calf bronchopneumonia. Design: Randomized controlled trial. Animals: 70 Holstein calves between one to six months old in three dairy farms of Mashhad suburb were used. Procedure: Calves showing visual signs of pneumonia were entered into the trial. After physical examination, clinical findings were recorded. They were weighed and blood samples collected by venipuncture. They were randomly assigned to received either Florfenicol (20mg/kg bw, IM, 2 injections 48h apart, 35 calves) or Tilmicosin (10mg/kg bw, SC, 1 injection, 35 calves). Clinical measures of efficacy included mortality, rectal temperature, illness index scores, assessment of treatment success or failure, and the number of relapses or reinfections. Also, hematological indices, total plasma protein and fibrinogen were studied on day 0 and day 4 after treatment. Stattistical analysis: Independent t-test, Mann_Whitney and Chi- Square. Results: Differences were not statistically significant between treatment groups (P>0.05). Resolving clinical sings in Florfenicol and Tilmicosin treatments were, respectively, 88.5% (31 cases) and 85.7 % (30 cases), which were statistically not significant (P>0.05). No significant adverse reactions were noticed with either drug. Conclusion: Results indicate that florfenicol and tilmicosin are comparable in the treatment of enzootic calf bronchopneumonia.

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