The critical behavior of a polymer blend was measured by small-angle neutron scattering above and below the critical temperature T-c, i.e., outside and inside the unstable region. The critical exponents and the amplitudes of the susceptibility and the correlation length were determined. For T > T-c the critical exponents gamma and nu are consistent with the renormalization-group predictions. Below T-c, however, deviations from the expected values were observed. Also experimental critical amplitude ratios (20-30)% smaller than predicted were found.