By examining the mechanical properties of isolated skeletal and cardiac myofibrils in calcium-free, ATP-containing solution, we attempted to separate the stiffness contribution of titin filaments from that of weakly bound cross bridges. Efforts to enhance weak cross-bridge binding by lowering ionic strength were met by clear contractile responses. Even at low temperature, myofibrils bathed in low-ionic-strength relaxing solution generated increased force and exhibited sarcomere shortening, apparently caused by active contraction. At normal ionic strength, myofibril stiffness, estimated from the force response to rapid sinusoidal oscillations, increased steadily with sarcomere extension up to a strain limit. No obvious stiffness contribution from weak cross bridges was detectable. Instead, the stiffness response, which was frequency dependent at all sarcomere lengths, was apparently generated by the viscoelastic titin filaments. During imposed stretch-hold ramps, both peak force/stiffness and the amount of subsequent stress relaxation increased with higher stretch rates, larger stretch amplitudes, and longer sarcomere lengths. We conclude that, for a truly relaxed myofibril, both passive force and dynamic stiffness principally reflect the intrinsic viscoelastic properties of the titin filaments.