  Relativistic effects in a three-body calculation of the $np\to\eta d$ process
are considered. Relativistic effects on the range and strength of the pion
exchange contribution to the reaction mechanism may be large, but boost effects
of the two-body interactions are negligible. The relativistic calculation
confirms previous non-relativistic results, showing that the shape of the cross
section near threshold is essentially determined by the $\eta d$ final-state
interaction alone. As for the region away from threshold, the relativistic pion
exchange contribution is seen to dominate the other mechanisms of the reaction.
It turns out that, within the relativistic reaction model, the $np\to\eta d$
experimental data excludes large values for the real part of the $\eta N$
scattering length.
