  An analysis of cosmological mixed dark matter models in spatially flat
Friedmann Universe with zero $\Lambda$-term is presented. We argue that the
introduction of cosmic gravity waves helps to satisfy observational
constraints. The analysis of models is based on the confrontation with the mass
function of clusters of galaxies and the CMB anisotropy. The implication of
Press-Schechter formalism allowed to constrain $\sigma_8=0.52 \pm 0.01$. This
normalisation of the spectrum of density perturbations has been used to
calculate numerically the value of the large scale CMB anisotropy and the
relative contribution of cosmological gravitational waves, T/S. We found that
increasing $\Omega_\nu$ weakens the requirements to the value of T/S, however
even for $\Omega_\nu\le 0.4$ the models with $h+n\ge 1.5$ suggest considerable
abundance of gravitational waves, T/S${}^>_\sim 0.3$. Minimisation of the value
T/S is possible only in the range of the red spectra ($n<1$) and small $h$
($<0.6$). However the parameter $\Omega_\nu$ is strongly constrained by $\Delta
T/T$ data on the first acoustic peak of Sakharov oscillations. Assuming that
T/S $\in[0,3]$ and taking into account observational data on the amplitude of
the first acoustic peak we constrain the model parameters. We show that the
considered models admit both moderate red and blue spectra of density
perturbations $n\in [0.9,1.2]$ with rather high abundance hot dark matter,
$\Omega_\nu \in [0.2,0.4]$. Any condition, $n<0.9$ or $\Omega_\nu<0.2$,
decreases the relative amplitude of the first acoustic peak for more than 30%
in comparison with its height measured by BOOMERanG.
