  By high temperature series expansion and exact complete diagonalization the
magnetic susceptibility chi(T) and the specific heat C(T) of a two-leg S=1/2
ladder with cyclic (4-spin) exchange are computed. Both methods yield
convincing results for not too small temperatures. We find that a small amount
of cyclic exchange influences the thermodynamical properties significantly. Our
results can serve as reliable basis for an efficient analysis of experimental
data
