Haptoglobin, a major bovine acute phase protein, was evaluated as a marker of the primary replication of foot-and-mouth disease virus in 12 naturally infected cattle from which blood was collected daily. An acute phase response, as measured by an increase in serum haptoglobin concentration and the presence of fever, was not detected during the previraemic stage of disease, but there was a significant increase in serum haptoglobin after the onset of viraemia. It occurred on the same day as the first appearance of clinical signs in three of the cattle, one day later in four and two days later in the other five cattle.