Serial determinations of C-reactive protein (CRP) were performed by single radial immunodiffusion in four groups of patients with fungal disease in order to determine its use as an aid to diagnosis. Elevated values (17-284 mg/l) of CRP were seen in 13 patients with fungal septicaemia before and/or within a few days after the first positive blood culture. C-reactive protein was not elevated in two patients with transient fungaemia. In 76 per cent (25/33) of deep-seated fungal infections in patients with malignant disorders of the blood CRP increased to 104-380 mg/l. In the remainder values of 36-92 mg/l were seen. In 37 per cent (19/51) of episodes of acute uncomplicated fungal stomatitis in patients with malignant disorders of the blood CRP rose to 110-320 mg/l. C-reactive protein was not found to be raised from the normal value (less than or equal to 6 mg/l) in 6 per cent of the episodes; in the rest, values below 100 mg/l were seen. CRP in 23 patients with acute fungal stomatitis but who were not immunocompromised remained normal. Thus, with regard to its ability to induce high (greater than 100 mg/l) CRP values, deep-seated fungal disease would seem to resemble bacterial rather than viral infection.