We have recently shown that antibody-induced blockade of C5a, C5a receptors, or IL-17A greatly reduced the harmful outcomes of sepsis. In the current study, normal cardiomyocytes from young (300 g) male Sprague-Dawley rats responded in vitro to C5a (ED₅₀=55 nM) with release of IL-6 and TNFα, peaking between 2 to 8 h. Neutralizing antibodies to mouse C5a or IL-17A (ED₅₀=40 μ g for each, based on improved survival) reduced spontaneous in vitro release of cardiosuppressive cytokines and chemokines in cardiomyocytes obtained from mice with polymicrobial sepsis. A non-neutralizing C5a antibody had no such effects. Cardiomyocytes from septic mice (C57Bl/6) showed increased mRNA for TNFR1, IL-6 (gp80), and C5aR at 6 h after sepsis. Cardiomyocytes from septic C5aR^−/− or C5L2^−/− mice did not show spontaneous in vitro release of cytokines and chemokines. These data suggest that cardiomyocytes from septic mice release suppressive cytokines in a C5a-, C5aR-, and IL-17A-dependent manner, followed by mediator reactivity with receptors on cardiomyocytes, resulting in defective contractility and relaxation. These data may be relevant to a strategy for the treatment of heart dysfunction developing during sepsis.—Atefi, G., Zetoune, F. S., Herron, T. J., Jalife, J., Bosmann, M., Chen, A., Al-Aref, R., Sarma, J. V., Ward, P. A. Complement dependency of cardiomyocyte release of mediators during sepsis.