Activation of the transcriptase in purified reovirus by chymotrypsin requires facilitating conditions in the digestion mix. In the presence of facilitation the virions are uncoated to cores. Without facilitation, uncoating is blocked at an intermediate stage, with transcriptase latent. Two independent types of facilitation have been discovered. The first type is mediated by K+, Rb+ or Cs+ ions. A second type, described here, is mediated by suitably high concentrations of virus in the digestion mix and requires collisions between intermediate subviral particles for its occurrence. The virus-concentration-mediated process is reversibly inhibited by high levels of active chymotrypsin.