Fibrillar granules (FGs) represent neutrophilic primary granules containing clustered filaments. We investigated neutrophils in the bone marrow obtained from 17 patients with chronic myeloproliferative disorders (CMPD) by electron microscopy. FG-positive neutrophils were seen in 15 of the 17 CMPDs with varying frequencies. The diameter of FG-filaments was 7-8 nm, showing that they corresponded to neither intermediate filaments nor actin filaments.