  The radiative ortho-para transition in the molecular hydrogen is studied.
This highly forbidden transition is very sensitive to relativistic and subtle
nonadiabatic effects. Our result for the transition rate in the ground
vibrational level $\Gamma(J=1\to J=0) = 6.20(62)\cdot 10^{-14} \iyr$ is
significantly lower in comparison to all the previous approximate calculations.
Experimental detection of such a weak line by observation of, for example, the
cold interstellar molecular hydrogen is at present unlikely.
