Long-term marrow cultures were established from 35 patients with aplastic anemia (AA) and the adherent stromal cell layers were assessed. Cultures from 23 of the 35 patients grew scanty stromal cell layers or did not produce any adherent cells. Long-term cultures from the remaining patients formed adherent cell layers that appeared to be morphologically normal. Cultures from 7 patients that grew apparently normal adherent cell layers were examined for expression of intermediate filament proteins using antibodies CGA-7 and HHF, which respectively recognize actin epitopes expressed in smooth muscle and normal marrow stromal cells. Cells from 4 of the 7 patients expressed vimentin (antibody 43 beta E8) but did not react with CGA-7 or HHF. It thus appears that most patients with AA have quantitative or qualitative abnormalities in the adherent cell layers from long-term marrow cultures, suggesting a defect in the hematopoietic microenvironment.