54 patients with advanced breast cancer were randomised into a prospective, non-blinded, controlled trial to receive: mitoxantrone 28 mg/m2 intravenous day 1 and granulocyte-colony stimulating factor (G-CSF) 5 micrograms/kg/day subcutaneously days 2 to 16 (n = 27) or the same regimen plus thymostimulin (TS) 50 mg/day intramuscular at days 2 to 16 (n = 27). The median time to reach a neutrophil count greater than 0.5 x 10(9)/l was lower in the G-CSF+TS treated group (9.13 versus 3.24 days; P < 0.0005). More patients experienced neutropenic fever in the G-CSF group than in the G-CSF+TS group (59.3% versus 22.2%, P = 0.0119). The incidence, duration and severity of clinically or bacteriologically documented infection were lower in patients who received TS. 16 patients (59.3%) in the G-CSF group contracted infection, and 4 patients (14.8%) receiving G-CSF+TS (P = 0.0016). These data indicate that the combination of G-CSF and TS is well-tolerated and may enhance haematological recovery following myelosuppressive chemotherapy in patients with advanced breast cancer.