Although recombinant human interleukin-3 (rhIL-3) shortens both the duration of chemotherapy-induced neutropenia and thrombocytopenia, its effect on nadir counts is limited. Concurrent administration of rhIL-3 and chemotherapy may enhance this effect. However, simultaneous administration of other hematopoietic growth factors and chemotherapy has resulted in enhanced myelosuppression. We investigated whether concomitant administration of rhIL-3 and chemotherapy would result in enhanced myelosuppression. Twelve patients with relapsed small cell lung cancer received vincristine, ifosfamide, mesna, and carboplatin on day 1 every four weeks. RhIL-3 was administered subcutaneously on days 1-14 during cycle 1 at doses of 4 (three patients) or 8 micrograms/kg/day (nine patients). During cycle 2 patients received only chemotherapy. No significant difference in leukocyte (1.4 +/- 1.0 vs. 0.9 +/- 0.4 x 10(9)/l (mean +/- SD), neutrophil (0.5 +/- 0.6 vs. 0.2 +/- 0.2 x 10(9)/l), and platelet (64 +/- 60 vs. 38 +/- 58 x 10(9)/l) nadir counts were demonstrated. The hemoglobin nadir level was significantly higher during cycle 1 (6.5 +/- 1.1 vs. 5.5 +/- 0.9 mmol/l, P = 0.05). Both leukocyte and platelet recovery were significantly enhanced in the rhIL-3 cycle. There was no significant difference in chemotherapy postponement or platelet transfusions. As a result of severe headaches, rhIL-3 administration was discontinued in one patient at 8 micrograms. RhIL-3 during this chemotherapy regimen for relapsed small cell lung cancer did not enhance myelotoxicity but did improve bone marrow recovery. This observation may increase the application of rhIL-3, for instance in combination with other hematopoietic growth factors.