Platelet-activating factor (PAF) is a phospholipid mediator of inflammation present in the human bone marrow. Freshly isolated human mononuclear bone marrow cells and marrow stromal cell cultures produced PAF under calcium ionophore (2 microM) and LPS (10 micrograms/ml) stimulation. By contrast, M-CSF (1000 U/ml), GM-CSF (100 ng/ml), IL1, IL3, IL6 and stem cell factor (10 ng/ml) did not stimulate PAF production. Marrow stromal cells produced 50-fold more PAF than freshly isolated mononuclear marrow cells, suggesting that stromal cells might be the major source of the human marrow-derived PAF. Mononuclear marrow cells and stromal cell cultures metabolized PAF with 1-alkyl-2-acyl-glycerophosphocholine as the major metabolic product. PMSF and p-BPB decreased the catabolism of PAF by freshly isolated marrow cells, but not by stromal cell cultures. While stromal cells rather than haematopoietic progenitors might be a major source of the human bone-marrow-derived PAF, both cell types metabolize it, suggesting their putative role in the regulation of PAF concentration in the human bone marrow.