The aim of this work was to characterize folates modified by ONOO- and HOCl and to evaluate the binding capacity of folates modified by ONOO-to folate receptor alpha and beta. For the modification of folate by ONOO-, folic acid was reacted with the combination of PMA activated PMN and PAPA NONOate or chemically synthesized ONOO-. For the modification of folate by HOCl, folic acid was reacted with the combination of MPO and H2O2 or NaOCl The structures of products were determined by H-1-NMR and MALDI-TOF mass. Nitrated folate species were identified as 10-nitro-folate and 12-nitro-folate, and chlorinated folate was identified as 12-chloro-folate. The 10-nitro-folate showed the selective binding to FR-beta, compared to folic acid. (C) 2002 Elsevier Science (USA). All rights reserved.