We have previously demonstrated an inverse relationship between circulating endogenous G-CSF levels and myeloid engraftment post-BMT. A new early-acting hematopoietic growth factor, Steel factor (SLF), has recently been demonstrated to induce the proliferation of early hematopoietic progenitor cells and synergistically stimulate committed progenitor cells in the presence of lineage-specific CSFs. In this pilot study, we determined the temporal relationship between endogenous SLF levels and the circulating absolute neutrophil count (ANC) (myeloid engraftment) in both children and adults undergoing both allogeneic and autologous BMT. Pre-BMT SLF levels were 2600 +/- 100 pg/ml compared to significantly lower levels of G-CSF (30-50 pg/ml). The circulating SLF level was significantly decreased throughout the post-BMT period (ANC < or = 200 x 10(6)/l: 1500 +/- 600 pg/ml; ANC 200-500 x 10(6)/l: 1780 +/- 130 pg/ml; ANC > or = 500 x 10(6)/l: 1690 +/- 110 pg/ml) (p < 0.001). There was a lack of an inverse relationship between the circulating SLF level and the ANC (r = -0.43) (p = NS). For comparison, SLF levels from immune thrombocytopenia (platelet < = or 20 x 10(9)/l) and chemotherapy-induced neutropenia patients (ANC < or = 200 x 10(6)/l) were similar to pre-BMT levels but significantly higher than post-BMT levels (p < or = 0.02 and < or = 0.001, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)