  We address the effect of the field induced antiferromagnetism in paramagnetic
state of the cuprate weak ferromagnet La$_2$CuO$_4$. The planar oxygen $^{17}$O
Knight shift is shown to be an effective tool to inspect the effects of
Dzyaloshinsky-Moriya coupling in cuprates in an external magnetic field. Field
induced antiferromagnetism and anisotropic antiferromagnetic contribution to
$^{17}$K explain the anomalies observed in $^{17}$O NMR in La$_2$CuO$_4$. The
experimental observation of antiferromagnetic contribution to the $^{17}$O
Knight shift provides probably the only way to find out the problem of the
sense of Dzyaloshinsky vector in cuprates.
