The purpose of this study was to establish the safety and tolerability of recombinant transforming growth factor-beta3 (TGF-beta3; CGP 46614) mouthwashes intended for prevention of chemotherapy-induced mucositis. Local effects were especially analyzed by objective and subjective measurements of mucositis. Secondary aims were analysis of potential systemic exposure and development of anti-TGF-beta3-antibodies. Eleven breast cancer patients received chemotherapy with 1.5 g/m2 cyclophosphamide i.v., 80 mg/m2 epirubicin i.v., and 1.0 g/m2 5-fluorouracil i.v. (n = 8) or 1.6 g/m2 carboplatin i.v., 480 mg/m2 thiotepa i.v., and 6 g/m2 cyclophosphamide i.v. divided over 4 days (n = 3). TGF-beta3 mouthwashes (10 ml; provided by Novartis, Basel, Switzerland) were administered for 4 days, four times a day, starting 1 day before chemotherapy. The dose was escalated in following patients from 25 microg/ml (n = 3) to 50 microg/ml (n = 3) and 100 microg/ml (n = 5). Clinically, the mucosa was scored objectively and according to WHO criteria. The percentage of viable oral epithelial cells was determined by trypan blue dye exclusion. Morphology of cells was assessed in buccal smears. Plasma samples were collected for determination of TGF-beta3 levels and anti-TGF-beta3-antibodies. Adverse events were recorded by the patient in a diary. Mouthwashes with TGF-beta3 were well tolerated. Three patients scored for mucositis > grade 0 (WHO grading criteria). The percentage of viable oral epithelial cells in patients treated with 1.5 g/m2 cyclophosphamide i.v., 80 mg/m2 epirubicin i.v., and 1.0 g/m2 5-fluorouracil i.v. was stable, whereas in patients treated with 1.6 g/m2 carboplatin i.v., 480 mg/m2 thiotepa i.v., and 6 g/m2 cyclophosphamide i.v. divided over 4 days, an increase was observed. The morphology of buccal cells showed a transient shift from mature to immature cells in the first week. Neither systemic absorption of TGF-beta3 nor development of TGF-beta3-antibodies was observed. TGF-beta3 mouthwashes were well tolerated and deserve further study in preventing chemotherapy-induced mucositis.