A prospective, randomized, double-blind, multicenter study in cancer patients receiving myelosuppressive chemotherapy was undertaken to evaluate and compare the tolerability of sargramostim (yeast-derived recombinant human granulocyte-macrophage colony-stimulating factor, RhuGM-CSF) and filgrastim (bacteria-derived recombinant human granulocyte colony-stimulating factor, RhuG-CSF) in the prophylaxis or treatment of chemotherapy-induced neutropenia. In all, 137 evaluable patients received sargramostim (300 micrograms; 193 mg/m2) or filgrastim (481 mg; 7 mg/kg) once daily by self-administered s.c. injection, usually beginning within 48 h after completion of chemotherapy. With the exception of a slightly higher incidence of grade 1 fever (< 38.1 degrees C) with sargramostim, there were no statistically significant differences in the incidence or severity of local or systemic adverse events possibly related to the growth factors. Although the study was not designed to evaluate efficacy directly, there also were no statistically significant differences between treatment groups in total days of growth factor therapy, days of hospitalization, or days of i.v. antibiotic therapy during the treatment period. Both sargramostim and filgrastim were comparably well tolerated when given by s.c. injection in this group of patients, and no clinically significant differences between the growth factors were demonstrated.