In order to further evaluate the effects of rGM-CSF on the reconstituting granulopoiesis, plasma and serum levels of myeloperoxidase (MPO) and lactoferrin (LF), as well as serum levels of eosinophil cationic protein (ECP), were monitored daily during a period of 3-4 weeks following ABMT in a group of 22 patients treated with either rGM-CSF (n = 11) or placebo (n = 11). Despite faster increase in the neutrophil counts in the rGM-CSF group, we did not observe any difference either in P-MPO or in P-LF during the period of early engraftment (days 11-19). This finding indicates that the proliferative effect of rGM-CSF on the neutropoiesis may be overestimated when neutrophil counts alone are taken into consideration, and suggests that other mechanisms may have contributed to the increase in the number of circulating neutrophils. The ratio of the serum to plasma level of LF, but not of MPO, was higher in the rGM-CSF group, probably reflecting a specific in vivo neutrophil priming effect. In the rGM-CSF group there was a clear increase of S-ECP during the second and third week post transplant, corresponding to an increase in eosinophil counts, which indicates that rGM-CSF stimulated eosinophil reconstitution without causing excessive activation of the mature eosinophils.