C-reactive protein and serum amyloid A protein levels were measured in 54 patients with pulmonary tuberculosis. The primary tuberculous complex was associated with an insignificant acute phase response, while post-primary tuberculosis without evidence of lung destruction caused modest increases in C-reactive protein and serum amyloid A protein. In most patients with post-primary pulmonary tuberculosis with significant pulmonary destruction there was a major acute phase response, with very high serum amyloid A protein and C-reactive protein levels. The response in these patients is most likely to be due to secondary bacterial infection in addition to infection by Mycobacterium tuberculosis. Patients with miliary tuberculosis showed a major acute phase response. Serum amyloid A protein and C-reactive protein levels decreased rapidly after initiation of treatment in the patients with post-primary tuberculosis without significant pulmonary destruction.