  We study the BPS counting functions (free energies) of the M-string
configurations. We consider separated M5-branes along with M2-branes stretched
between them, with M5-branes acting as domain walls interpolating different
configurations of M2-branes. We find recursive structure in the free energies
of these configurations. The M-string degrees of freedom on the domain walls
are interpreted in terms of a pair of interacting supersymmetric WZW models. We
also compute the elliptic genus of the M-string in a toy model of the ABJM
theory and compare it with the M-theory computation.
