The results are presented of two preliminary studies conducted to assess the role of GM-CSF as adjuvant treatment in neutropenic patients with bacterial or fungal infections. In the first study the effect of GM-CSF on the rate of response to antibiotics was assessed. Febrile neutropenic patients (n = 91) were randomized to receive ticarcillin-clavulanate plus netilmicin with or without GM-CSF (60 micrograms/m2). Response rates were significantly higher in patients who received antibiotics plus GM-CSF (p = 0.05). An increase in neutrophil count was seen in 89% of GM-CSF patients with initial low neutrophil counts (< 100/microliters) compared with 67% of control patients (p = 0.04). In the second study the activity of GM-CSF in patients with fungal infections was assessed. Neutropenic patients with documented fungal infections received amphotericin B plus GM-CSF. Of the eight evaluable patients, six responded and four had a complete response to treatment. The neutrophil counts of the two non-responding patients did not increase substantially during GM-CSF treatment and both died of their fungal infection. The prognosis of neutropenic patients with fungal infections is usually poor and the results of this pilot study are therefore very encouraging. The two studies show that GM-CSF is able to stimulate neutrophil recovery in neutropenic patients and may improve the response to antibiotic and antifungal treatment.