The marrows of 10 patients with hematologic malignancies were examined by immunohistochemistry using anti TGF-β antibody, CC(l-30), which detects secreted TGF-β, and compared with four normal marrows. TGF-β was not demonstrated in marrows with a normal level of rcticulin fibrosis; however, TGF-β was observed within collagen in marrows having collagen fibrosis or increased reticulin fibrosis. The extent of TGF-β deposition paralleled the severity of fibrosis (P < .0001), and occurred even with normal or reduced numbers of megakaryocytes. Using another TGF-β antibody, LC(l-30), which detects intracellular TGF-β, TGF-β was detected by immunofluorescence in discrete sites in the cytoplasm of immature and mature myeloid and large granular lymphocytic leukemia cells. These sites colocalized with areas detected by an anti-granule antibody (D545) suggesting that TGF-β was stored in granules. However, neither the TGF-β mRNA content nor the degree of TGF-β secretion by these leukemic cells correlated with the extent of TGF-β deposition in the marrow. Thus, TGF-β deposition in marrow may contribute to myelofibrosis, but the source of this cytokine in the absence of megakaryocytes requires further study.