Our group previously demonstrated a strong association between elevated plasma soluble CD13 enzyme activity and newly diagnosed extensive chronic GVHD (cGVHD) in children. As cytotoxic anti-CD13 Abs have been documented after blood and marrow transplant (BMT) in association with CMV infection and cGVHD, we hypothesized that soluble CD13 contributes to cGVHD pathogenesis by induction of CD13 reactive Abs and that anti-CD13 Abs could be additional biomarkers for newly diagnosed pediatric extensive cGVHD. Using prospectively collected plasma samples from pediatric allogeneic BMT (allo-BMT) subjects with cGVHD and controls without cGVHD enrolled in a large multi-institution Children's Oncology Group cGVHD therapeutic trial, we evaluated whether soluble CD13 correlates with induction of anti-CD13 Abs. We found that CD13 reactive Abs are present in a proportion of patients after allo-BMT, but did not seem to correlate with the presence of soluble CD13. Anti-CD13 Abs also did not meet our criteria as a diagnostic biomarker for cGVHD. These data do not confirm that induction of CD13 reactive Abs is a mechanism for cGVHD in children nor are part of the pathogenesis of cGVHD associated with elevated soluble CD13. The exact role of CD13 in cGVHD remains to be determined. © 2010 Macmillan Publishers Limited. All rights reserved.