A new technique for injecting scalar fluctuations in a DNS of isotropic turbulence is presented. It is used to study statistically steady states associated with different levels of mixing. The results are analysed in terms of p.d.f and they are used as a data base to investigate the effect of the filtering operation that is performed in the case of a LES. It is shown that the p.d.f of the scalar is substantially affected by the filtering operation. It is also shown that the Cook and Riley (1994) subgrid model allows to reconstruct a p.d.f which is in fairly good agreement with the unfiltered DNS results.