Membrane penetration by reovirus is associated with conversion of a metastable intermediate, the ISVP, to a further-disassembled particle, the ISVP*. Factors that promote this conversion in cells are poorly understood. Here, we report the in vitro characterization of a positive-feedback mechanism for promoting ISVP* conversion. At high particle concentration, conversion approximated second-order kinetics, and products of the reaction operated in trans to promote the conversion of target ISVPs. Pore-forming peptide mu1N, which is released from particles during conversion, was sufficient for promoting activity. A mutant that does not undergo mu1N release failed to exhibit second-order conversion kinetics and also failed to promote conversion of wild-type target ISVPs. Susceptibility of target ISVPs to promotion in trans was temperature dependent and correlated with target stability, suggesting that capsid dynamics are required to expose the interacting epitope. A positive-feedback mechanism of promoting escape from the metastable intermediate has not been reported for other viruses but represents a generalizable device for sensing a confined volume, such as that encountered during cell entry.