The lectin binding pattern of bone marrow cells in normal and reactive states and in various neoplastic disorders was investigated using trephine biopsy specimens taken from the iliac crest. The tissue samples were routinely processed (fixed in formalin and embedded in paraffin wax) and subjected to mild decalcification with EDTA. The following results were obtained. (1) More than half of the 23 fluoresceinated lectins used reacted with normal blood cells and/or their neoplastic derivatives. Inhibition tests with the appropriate sugars confirmed the specificity of binding for the majority, but not all, of the lectins. (2) WGA, Con A, PSA, STA and RCA60 and RCA120 produced a particularly intense reaction with normal, reactive and neoplastic myeloid cells. Erythroblasts exhibited weak staining in a few cases by a few lectins (WGA producing the strongest staining), while megakaryocytes nearly always remained unstained. Neoplastic lymphoid cells in various lymphoproliferative disorders and plasmacytoma cells generally reacted with the same lectins as the myeloid cells. (3) Since neoplastic myeloid cells in various myelodysplastic and myeloproliferative disorders exhibited a lectin binding pattern similar to that of myeloid cells in normal and reactive bone marrow, it is unlikely that lectin histochemistry of the bone marrow will prove of great value in the diagnosis of myelodysplastic-myeloproliferative disorders.