Let \(\Omega=[0,1]\times[0,1]\) and let \(C^{1,1}_ \omega(\Omega)\) denote a space of bivariate functions with continuous partial derivatives of order one on \(\Omega\) with \(\omega(f;t,\tau)\leq\omega(t,\tau)\). Here, \(\omega(t,\tau)\) denotes a convex modulus of continuity and \(\omega(f;t,\tau)\) stands for the usual modulus of continuity of \(f\). In the paper under review the author gives an exact value of \(\hbox{Sup}_{f\in c^{1,1}_ (\omega)}\| f^{(1,1)}- S^{(1,1)}_{1,1}\|_ \infty\), where \(S_{1,1}\) is the bilinear spline which interpolates \(f\) at the points \((x_ i,y_ j)\) \(x_ i=i/n\), \(y_ j=j/m\) (\(0\leq i\leq m\), \(0\leq j\leq m\)) and \(\|\cdot\|_ \infty\) stands for the sup-norm over \(\Omega\).