A direct comparison of recombinant tumor necrosis factor (rTNF) and the monocyte-derived cytotoxic factor (CF) which is involved in monocyte-mediated cytotoxicity revealed immunological, physiochemical, and biological similarities, indicating that TNF is an effector molecule in monocyte-mediated cytotoxicity. Neutralizing antiserum raised against rTNF completely inhibited the ability of CF-containing monocyte supernatants to induce cytolysis and cell death of sensitive target cells and, conversely, antiserum raised against purified CF completely inhibited the cytotoxic activity of rTNF. Both CF and rTNF have an apparent isoelectric point of 5.8-5.9 as determined by chromatofocusing, and a molecular weight of about 40,000 as determined by gel filtration. Moreover, when present in monocyte supernatants with a total protein concentration of about 1 mg/ml and 0.1% sodium dodecyl sulfate, both CF and rTNF migrated with a molecular weight of about 35,000 upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Pure rTNF, however, migrated with a molecular weight of 17,000, suggesting that the relative amount of sodium dodecyl sulfate to protein is critical for dissociating the apparent dimeric structure of TNF. CF and rTNF were also similar with respect to their ability to kill various types of target cells the sensitivity of which to TNF differ, and the dose-response curves of cytotoxicity obtained with CF-containing monocyte supernatants and rTNF were similar. As is the case with anti-CF serum, anti-rTNF serum inhibited drug-dependent cellular cytotoxicity and cytolysis mediated by both freshly isolated monocytes and in vitro cultured unactivated and lymphokine-lipopolysaccharide activated monocytes, indicating that TNF is an effector molecule in both drug-dependent cellular cytotoxicity and "classical" monocyte-mediated cytotoxicity.