Transforming growth factor-beta 1 (TGF-beta 1) is a multifunctional cytokine which promotes fibroblast growth and collagen synthesis, but suppresses growth and differentiation of immune lymphocytes and killer cells. Immunohistochemical detection of TGF-beta 1 in Hodgkin's disease (HD) has been shown to correlate with the histologic feature of nodular sclerosis, which is associated with a favorable prognosis (American Journal of Pathology 1990, 136:1209). In that study, TGF-beta 1 was localized mainly at the margins of broad collagen bands (presumably sites of new collagen synthesis) and in areas containing numerous Hodgkin/Reed-Sternberg cells (H/RS). In these areas, TGF-beta 1 protein was found on the membrane and occasionally within the cytoplasm of H/RS cells. To determine whether TGF-beta 1 is synthesized by H/RS cells or secondarily bound to their membrane and sometimes internalized, we performed in situ hybridization (ISH) using 1.5 Kb 35S-labeled anti-sense and sense RNA probes to TGF-beta 1. Paraffin-embedded tissues of 10 cases from all histologic types of HD were examined. Somewhat unexpectedly, the major site of TGF-beta 1 mRNA was in eosinophils; TGF-beta 1 mRNA was not detected in H/RS cells. TGF-beta 1 mRNA was found in eosinophils in all cases of nodular sclerosis but not in other types of HD, despite the presence of numerous eosinophils in mixed cellularity cases. The presence of TGF-beta 1 mRNA coincided with immunohistochemical detection of TGF-beta 1 protein using antibody CC (1-30). These results confirm the role of TGF-beta 1 in the histogenesis of nodular sclerosing HD and indicate that eosinophils are the major source of TGF-beta 1 in this type of HD.