Summary. Patients are at risk of mucositis and infections in the oral cavity during the neutropenic period after chemotherapy, which are significant causes of morbidity. In phase I/II studies with the haemopoietic growth factor granulocyte colony stimulating factor (G-CSF), a reduction in post-chemotherapy mucositis has been observed in addition to haematologic effects. To understand this phenomenon better in patients receiving G-CSF following high-dose chemotherapy with autologous bone marrow transplantation (ABMT), we studied the effects of G-CSF on levels of neutrophils recoverable from the oral cavity using a quantitative mouthrinse assay. In normal subjects, mouthrinses contained 472 ± 329 ± 103 neutrophils/mouthrinse. After chemotherapy followed by ABMT, mouthrinse neutrophil levels decreased to undetectable levels during the neutropenic period, but recovered 1–2 and 3–9 d before circulating neutrophil levels reached 0 ± 1 and 1 ± 109/1 respectively, whether or not patients received G-CSF. In patients who received G-CSF, the mean cumulative mucositis score was reduced from 35 ± 9 to 21 ± 12 (P < 0 ± 05), and the maximum mean daily mucositis score was reduced from 2·8 ± 0·5 to l·7 ± 0·9 (P < 0·01), compared to patients who did not receive G-CSF after ABMT. These studies provide in vivo evidence that neutrophils produced during G-CSF therapy are available to leave the circulation and enter tissues where their function is required for host defence. Since the usual temporal relationship between oral and peripheral blood neutrophil recovery was preserved during G-CSF administration after ABMT, these data support the hypothesis that the reduction in post-ABMT mucositis observed with G-CSF therapy may reflect a beneficial effect of G-CSF on the kinetics of oral mucosal neutrophil recovery in addition to the effect of G-CSF to accelerate peripheral blood neutrophil recovery.