Skinned skeletal and cardiac muscle fibres can be activated by MgADP in the presence of MgATP without Ca2+; the isometric tension is developed in a sigmoidal manner with the addition of MgADP under relaxing conditions. The critical concentrations of MgADP for this MgADP-induced contraction are about 7.5 and 2.6 mM for skeletal and cardiac muscle fibres, respectively. To investigate whether muscle regulatory proteins, myosin light chain 2 (LC2) and troponin C (TnC), play a part in the MgADP-induced contraction, these proteins were partly extracted by treatment with trans-1,2-cyclohexanediamine-N,N,N',N'-tetraacetic acid (CDTA), a chelater of divalent cations, and the MgADP-tension relationship was examined in rabbit psoas and bovine cardiac skinned fibres. We found that the sigmoidal MgADP-tension relationship became hyperbolic after a partial extraction of LC2 (about 30 %) and TnC (about 70 %). Reconstitution with LC2 restored the sigmoidal MgADP-tension relationship of control fibres almost fully in both skeletal and cardiac fibres, whereas reconstitution with TnC alone had no effect. Furthermore, cardiac fibres reconstituted with skeletal LC2 exhibited an MgADP-tension relationship intermediate between skeletal and cardiac fibres. The partial extraction of LC2 and TnC resulted in a reduction of the inhibitory effect of inorganic phosphate (P(i)) on the MgADP-activated tension. Reconstitution with LC2 restored the original P(i)-tension relationship, whereas reconstitution with TnC had no effect. In other words, extraction of LC2 apparently increased the affinity of myosin for MgADP but decreased the affinity for P(i). These results demonstrate that LC2 modulates MgADP-induced activation of actomyosin interaction.