Spectral line index data for a sample of 290 E and S0 galaxies are used to investigate the stellar populations of these galaxies .
250 of the galaxies are members of 11 nearby clusters ( cz _ { CMB } < 11500 km s ^ { -1 } ) .

It is studied how the stellar populations of the galaxies are related to the velocity dispersions , the masses of the galaxies , and the cluster environment .
This is done by establishing relations between these parameters and the line indices { { Mg } _ { 2 } } , { < \hskip { -1.5 pt } { Fe } \hskip { -1.5 pt } > } and { { H } { \beta } _ { G } } .
The difference between the slope of the { { Mg } _ { 2 } } - \sigma relation and the slope of the { < \hskip { -1.5 pt } { Fe } \hskip { -1.5 pt } > } - \sigma relation indicates that the abundance ratio [ Mg/Fe ] is 0.3-0.4 dex higher for galaxies with velocity dispersions of 250 km s ^ { -1 } compared to galaxies with velocity dispersions of 100 km s ^ { -1 } .
This is in agreement with previous estimates by Worthey et al .

The { < \hskip { -1.5 pt } { Fe } \hskip { -1.5 pt } > } index is stronger correlated with the projected cluster surface density , \rho _ { cluster } , than with the galaxy mass or the velocity dispersion .
We have earlier found the residuals for the { { Mg } _ { 2 } } - \sigma relation to depend on the cluster environment .
Here we determine how both the { { Mg } _ { 2 } } index and the { < \hskip { -1.5 pt } { Fe } \hskip { -1.5 pt } > } index depend on the velocity disperson and \rho _ { cluster } .
Alternative explanations that could create a spurious environment dependence are discussed .
No obvious alternatives are found .
The environment dependence of the { { Mg } _ { 2 } } - \sigma relation is supported by data from Faber et al .
The dependence on the environment implies that [ Mg/Fe ] decreases with increasing density , \rho _ { cluster } .
The decrease in [ Mg/Fe ] is 0.1 dex over 2.5 dex in \rho _ { cluster } .

We have also studied to what extend the mass-to-light ( M/L ) ratios of the galaxies are determined by the stellar populations .
The M/L ratios are strongly correlated with the indices { { Mg } _ { 2 } } and { { H } { \beta } _ { G } } , while the { < \hskip { -1.5 pt } { Fe } \hskip { -1.5 pt } > } index is only weakly correlated with the M/L ratio .

Based on current stellar population models we find that it is not yet possible to derive unique physical parameters ( mean age , mean abundances , mean IMF , and fraction of dark matter ) from the observables ( line indices , velocity dispersion , mass , M/L ratio ) .