Abstract The ability of human mononuclear cells (MNC) to produce cytokines is a highly sensitive and biologically relevant test system for the presence of microbiol products. The safety of parenteral fluids is presently determined by gelation of the limulus amebocyte lysate (LAL) to endotoxin. In the present study, crude bacterial culture supernatants from Escherichia coli were subjected to ultrafiltration using polysulfone and the ultrafiltrates were tested for their ability stimulate human MNC. Total interleukin-1 (IL-1) and tumor necrosis factor (TNF) produced by MNC were measured by radioimmunoassay. Endotoxin-like substances in E. coli cultures are rejected by a factor of at least 100 000. Rejection takes place by molecular size exclusion and by absorption. The sensitivity of the LAL and MNC cytokine production were comparable. These studies demonstrate a wide margin of safety for the production of parenteral fluids using ultrafiltration for endotoxin-containing materials.