  A stochastic background of primordial gravitational waves may substantially
contribute, via the Sachs--Wolfe effect, to the large--scale Cosmic Microwave
Background (CMB) anisotropies recently detected by COBE. This implies a {\it
bias} in any resulting determination of the primordial amplitude of density
fluctuations. We consider the constraints imposed on $n<1$ (``tilted")
power--law fluctuation spectra, taking into account the contribution from both
scalar and tensor waves, as predicted by power--law inflation. The
gravitational--wave contribution to CMB anisotropies generally reduces the
required {\it rms} level of mass fluctuation, thereby increasing the linear
{\it bias parameter}, even in models where the spectral index is close to the
Harrison--Zel'dovich value $n=1$. This ``gravitational--wave bias" helps to
reconcile the predictions of CDM models with observations on pairwise galaxy
velocity dispersion on small scales.
