Purpose: To compare the relative hematopoietic protective effects of recombinant human interleukin-1 a (rhuIL-lα), recombinant human granulocyte macrophage colony-stimulating factor (rhuGM-CSF), and PIXY321, a genetically engineered fusion protein combining interleukin-3 and rhuGM-CSF, in children with refractory solid tumors after treatment with ifosfamide, carboplatin, and etoposide (ICE).Patients and Methods: A total of 53 children who had not responded to at least one earlier chemotherapy regimen were enrolled on consecutive trials of ICE chemotherapy alone (n = 14) or with rhuGM-CSF (n = 8), rhuIL-lα (n = 10), or PIXY321 (n = 21). The relative hematopoietic effects of these three cytokines were compared retrospectively to each other and to values for patients who received ICE alone. Because one cannot assume that hematopoietic toxicity and response to a given cytokine are independent of the course of chemotherapy, the analysis was restricted to the first treatment course.Results: In this retrospective comparison, 1000 μg/m2/day of rhuGM-CSF reduced the median duration of grade 4 neutropenia (<500/μ.L) from a median of 17 days (range 3 to 34) in children who received ICE alone to 9 days (range 5 to 11, p = 0.003); it appeared to have a beneficial effect on severe thrombocytopenia (<20,000/μL), reducing the median duration from 4.5 days with ICE alone to 3 days (p = 0.08) and the number of platelet transfusions from a median of 5.75 transfusions (range 0 to 13) to 0 in these two cohorts. No significant improvement in these measures was seen with rhuIL-lα or PIXY321.Conclusions: This analysis suggests that 1000 μg/m2/day of rhuGM-CSF has clinically significant effects on platelet recovery and more effectively ameliorates thrombocytopenia and neutropenia than either rhuIL-1α or PIXY321 in the context of ICE chemotherapy. Further dose-intensification will require a combination of cytokines; the optimal dose and combination of these agents awaits further study.