C-reactive protein, measured in serum from 38 patients with leukaemia, was elevated to at least 100 mg/l at the beginning of 32 of 34 episodes of infection, and subsequently rose above 100 mg/l in all 34. Uninfected patients, whether in leukaemic remission or relapse and whether pyrexial or not, always had levels below 100 mg/l, with four exceptions out of 290 measurements. Estimation of two other acute-phase proteins, alpha 1-antitrypsin and orosomucoid, was not of additional diagnostic value. Serial measurement of C-reactive protein may be important for the early detection of infection in the leukaemic patient with neutropenia.