Abstract—We studied the effect of titin-based passive force on the length dependence of activation of cardiac myocytes to explore whether titin may play a role in the generation of systolic force. Force-pCa relations were measured at sarcomere lengths (SLs) of 2.0 and 2.3 &mgr;m. Passive tension at 2.3 &mgr;m SL was varied from ≈1 to ≈10 mN/mm2by adjusting the characteristics of the stretch imposed on the passive cell before activation. Relative to 2.0 &mgr;m SL, the force-pCa curve at 2.3 &mgr;m SL and low passive tension showed a leftward shift (&Dgr;pCa50[change in pCa at half-maximal activation]) of 0.09±0.02 pCa units while at 2.3 &mgr;m SL and high passive tension the shift was increased to 0.25±0.03 pCa units. Passive tension also increased &Dgr;pCa50at reduced interfilament lattice spacing achieved with dextran. We tested whether titin-based passive tension influences the interfilament lattice spacing by measuring the width of the myocyte and by using small-angle x-ray diffraction of mouse left ventricular wall muscle. Cell width and interfilament lattice spacing varied inversely with passive tension, in the presence and absence of dextran. The passive tension effect on length-dependent activation may therefore result from a radial titin-based force that modulates the interfilament lattice spacing.