Human fibroblastoid cell strains obtained from primary bone marrow cultures and continuous stromal cell lines recently derived from mouse bone marrow were studied. The incidence of fibroblastoid precursors (CFU-F) varied considerably in human bone marrow samples, and no differences could be detected between marrows from a group of myelodysplastic patients (age range 70-82 years) and groups of age-matched controls or younger individuals. A lack of direct correlation between initial clonogenicity and ultimate capacity of fibroblastoid cells to grow in continuous culture was observed in both the normal and the myelodysplastic groups. Despite the apparently normal clonogenicity of CFU-F in patients with myelodysplastic syndromes, some of these marrows failed to grow when subcultured. Normal fibroblastoid cells at 10(4) per culture exhibited myelopoietic activities when cocultured with fresh bone marrow cells. At higher concentrations, these cells inhibited myeloid colony formation. Fibroblastoid cells from only one out of four myelodysplastic patients examined exhibited comparable inhibitory activity. The specificity of the inhibitor(s) was demonstrated by the lack of effect of fibroblastoid cells from normal human bone marrow on the clonogenicity of mouse erythroleukemia cells. Moreover, human foreskin fibroblasts were devoid of such inhibitory activity. These functions of cultured stromal cells may correlate with some of their activities in the bone marrow microenvironment.