In the present study, the concentrations of serum amyloid A and surfactant protein D in sera were measured to evaluate them for identification of the clinical condition of horses with bacterial pneumonia. The study utilized 185 clinically healthy control thoroughbreds and 9 thoroughbreds for experimental infectious study with S. zooepidemicus. Blood samples were collected from the 185 healthy control thoroughbreds. The 9 thoroughbreds were experimentally infected S. zooepidemicus using an endoscopic injection to a lung lobe and were then observed of clinical conditions. Blood samples were collected before inoculation and on the 1-15th, 22nd, and 29th days after inoculation (follow-up group). The levels of SAA and SP-D in the healthy control thoroughbreds were very low. In the follow-up group, the levels of SAA and SP-D changed in parallel with the horses' clinical condition. The pyrexia observed after bacterial inoculation faded by the 11th day, and the changes in SAA and SP-D occurred simultaneous to disappearance of the clinical signs. Measurement of SAA and SP-D proved useful for monitoring the clinical condition of the horses with bacterial pneumonia. Changes in the SP-D value were preceded by changes in the SAA value. Since the changes in SP-D occurred approximately simultaneous to the changes in the horses' clinical signs, we believe that they reflect the condition of the alveolar membranes. We conclude that measurement of SAA and SP-D in sera is useful for identification of the clinical condition of horses with bacterial pneumonia.