Serum endotoxin, interleukin-6 (IL-6) and C-reactive protein (CRP) were serially determined in 26 patients with hematological malignancies and chemotherapy-induced neutropenia who developed fever. Endotoxin in serum was detected in 69% of the patients, with the highest values being recorded in patients with gram-negative (Gr-) bacteremia. High levels of IL-6 were found after start of fever, and in 6/9 patients with Gr- bacteremia levels exceeded 200 ng/l in samples drawn within the first 72 hours. However, only in 2/17 patients with gram-positive bacteremias and blood culture-negative fever episodes did IL-6 exceed this concentration (p < 0.05). High CRP values (exceeding 100 mg/l) did not discriminate between Gr- and non-Gr- episodes (7/9 versus 10/17, respectively). In patients with fever at day 3-5 (n = 15), IL-6 values > 100 ng/l were associated with fever continuing for more than 7 days after start of the episode; contrarily, CRP values did not indicate the persistence of fever. Determination of IL-6 may be a better test than CRP in monitoring the acute response to infection in the neutropenic patient. A combination of high endotoxin and IL-6 values may indicate a Gr- bacteremia. This could have therapeutic implications before results of blood cultures are obtained.