  We investigate properties and the distribution of light nuclei (A<4) in
symmetric nuclear matter of finite temperature within a microscopic framework.
For this purpose we have solved few-body Alt-Grassberger-Sandhas type equations
for quasi-nucleons that include self-energy corrections and Pauli blocking in a
systematic way. In a statistical model we find a significant influence in the
composition of nuclear matter if medium effects are included in the microscopic
calculation of nuclei. If multiplicities are frozen out at a certain time (or
volume), we expect significant consequences for the formation of light
fragments in a heavy ion collision. As a consequence of the systematic
inclusion of medium effects the ordering of multiplicities becomes opposite to
the law of mass action of ideal components. This is necessary to explain the
large abundance of $\alpha$-particles in a heavy ion collision that are
otherwise largely suppressed in an ideal equilibrium scenario.
