The serum concentration of C-reactive protein (CRP), a nonspecific acute phase reactant which responds sensitively to bacterial infection, was measured by homogeneous enzyme immunoassay in all patients admitted to a general hospital geriatric unit. It was then monitored frequently in those in whom infection was suspected. Results were reported rapidly to the clinical team and particularly in the context of a patient population with complex multisystem pathology, often without pyrexia, leucocytosis or other classical signs of infection, they made a significant contribution to patient management. Very high CRP levels were most commonly due to infection (75% of those over 50 mg/l and 94% of those over 100 mg/l) and the pattern of the CRP response during antimicrobial therapy provided a sensitive, objective index of its efficacy. Failure of CRP levels to fall promptly stimulated additional diagnostic investigations, revealing resistant infection, localized pus or serious noninfective pathology, particularly malignancy, and led to appropriate management. Furthermore the CRP results on admission were of considerable prognostic significance, being significantly higher (median 70 mg/l) in those patients who did not survive than in those who did (18 mg/l).