Previous studies have shown that the haemopoietically active murine long-term bone marrow culture (LTBMC) is a useful model for studying drug-induced suppression and recovery of myelopoiesis. We studied the effects on stromal morphology and stromal progenitors, assessed as colony forming unit-fibroblasts (CFU-F), of the addition of either the antimetabolite methotrexate (MTX) or the betalactam ceftazidime (CEF) to LTBMC. The examination of 500 microgram/ml CEF-treated cultures revealed a stroma that appeared empty, with modest reduction in total stromal counts, and significant decreases in fat-containing and endothelial cells. In contrast, treatment with 1 micromolar MTX for 1 week initially caused minimal morphologic stromal changes, thereafter total stromal cell count as well as fibroblastoid, endothelial, fat containing and macrophage cells significantly increased. Haemopoiesis and the stroma recovered. Both CEF and MTX reversibly suppressed stromal progenitor cells in LTBMC. When added directly to CFU-F cultures, the concentrations resulting in a 50 percent colony growth inhibition were 214 microgram/ml for CEF and 375 nM for MTX. These results suggest that CEF, but not MTX, has direct toxic effects on the stroma of established LTBMC. Stromal cell increase following MTX treatment probably indicates a stromal response that may contribute to haemopoietic cell recovery.