Thin filament regulation of muscle contraction is believed to be mediated by both Ca 2 + and strongly bound myosin cross-bridges. We found that secophalloidin (SPH, 5-8 mM) activates cross-bridge cycling without Ca 2 + causing isometric force comparable to that induced by Ca 2 + . At saturated [SPH], Ca 2 + further increased force by 20%. SPH-induced force was reversible upon washing with a relaxing solution. However, there was more than 30% irreversible loss in subsequent Ca 2 + -activated force. We hypothesize that SPH activates muscle via strongly bound cross-bridges. SPH-activated contraction provides a new model for studying the role of Ca 2 + and cross-bridges in muscle regulation.