  We investigate the impact of the existence of a primordial magnetic field on
the filter mass, characterizing the minimum baryonic mass that can form in dark
matter (DM) haloes. For masses below the filter mass, the baryon content of DM
haloes are severely depressed. The filter mass is the mass when the baryon to
DM mass ratio in a halo is equal to half the baryon to DM ratio of the
Universe. The filter mass has previously been used in semianalytic calculations
of galaxy formation, without taking into account the possible existence of a
primordial magnetic field. We examine here its effect on the filter mass. For
homogeneous comoving primordial magnetic fields of $B_0 \sim 1$ or 2 nG and a
reionization epoch that starts at a redshift $z_s=11$ and is completed at
$z_r=8$, the filter mass is increased at redshift 8, for example, by factors
4.1 and 19.8, respectively. The dependence of the filter mass on the parameters
describing the reionization epoch is investigated. Our results are particularly
important for the formation of low mass galaxies in the presence of a
homogeneous primordial magnetic field. For example, for $B_0\sim 1\nG$ and a
reionization epoch of $z_s\sim 11$ and $z_r\sim7$, our results indicate that
galaxies of total mass $M\sim5 \times 10^8\msun$ need to form at redshifts
$z_F\gtrsim 2.0$, and galaxies of total mass $M\sim10^8\msun$ at redshifts
$z_F\gtrsim 7.7$.
