Previous studies have demonstrated the presence of myocardial depression in clinical and experimental septic shock. This depression is associated with the presence of a circulating myocardial depressant substance with physical characteristics consistent with cytokines. The present study utilized an in vitro myocardial cell assay to examine the role of various human recombinant cytokines, including tumor necrosis factor (TNF)alpha and interleukin (IL)1beta, in depression of cardiac myocyte contractile function induced by serum from humans with septic shock. The extent and velocity of electrically paced rat cardiac myocytes in tissue culture was quantified by a closed loop video tracking system. Individually, TNF-alpha and IL-1beta each caused significant concentration-dependent depression of maximum extent and peak velocity of myocyte shortening in vitro. In combination, TNF-alpha and IL-1beta induced depression of myocardial cell contractility at substantially lower concentrations consistent with a synergistic effect. Using immunoabsorption, removal of both TNF-alpha and IL-1beta (but not either alone) from the serum of five patients with acute septic shock and marked reversible myocardial depression resulted in elimination of serum myocardial depressant activity. IL-2, -4, -6, -8, -10, and interferon gamma failed to cause significant cardiac myocyte depression over a wide range of concentrations. These data demonstrate that TNF-alpha and IL-1beta cause depression of myocardial cell contraction in vitro and suggest that these two cytokines act synergistically to cause sepsis-associated myocardial depression in humans.