Chaen et al. (1986. J. Biol. Chem. 261:13632–13636) showed that treatment of relaxed single muscle fibers with para-phenylenedimaleimide (pPDM) results in inhibition of a fiber's ability to generate active force and a diminished ATPase activity. They postulated that the inhibition of force production was due to pPDM's ability to prevent crossbridges from participating in the normal ATP hydrolysis cycle. We find that the crossbridges produced by pPDM treatment of relaxed muscle cannot bind strongly to the actin filaments in rigor, but do bind weakly to the actin filaments in the presence and also absence of ATP. After pPDM treatment, fiber stiffness, as measured using ramp stretches of varying duration, is ATP-insensitive and identical to that of untreated relaxed fibers (both at high [165 mM] and low [40 mM] ionic strength). These results suggest that the pPDM-treated crossbridges, in both the presence and absence of ATP, are locked in a state that resembles the weakly-binding myosin ATP state of normal crossbridges. Their resemblance to the ATP-crossbridges of relaxed untreated fibers is quite strong; both bind to actin about equally tightly and have similar attachment and detachment rate constants. We also found that crossbridges are locked in a weakly-binding state after treatment with N-phenylmaleimide (NPM). In muscle fibers, this method of producing weakly-binding crossbridges appears preferable to pPDM treatment because, unlike treatment with pPDM, it does not increase the fiber's resting tension and stiffness and it does not disrupt the titin band seen on SDS-PAGE.