  Context: The redshift of PG 1553+11, a bright BL Lac object (V~14), is still
unknown. It has been recently observed in the TeV band, a fact that offers an
upper limit for the redshift z<0.4. Aims: We intend to provide a lower limit
for the distance of the object. Methods: We used a chi^2 procedure to constrain
the apparent magnitude of the host galaxy in archived HST images. Supposing
that the host galaxy is typical of BL Lac objects (M_{R} -22.8), a lower limit
to the distance can be obtained from the limit on the apparent magnitude of the
host galaxy. Results: Using the 3 sigma limit on the host galaxy magnitude, the
redshift is found to be greater or equal to 0.25. Conlusions: The redshift of
PG 1553+11 is probably in the range z=0.3-0.4, making this object the most
distant extragalactic source so far detected in the TeV energies. We suggest
that other bright BL Lac objects of unknown redshift and similar spectroscopic
characteristics may be interesting targets for TeV observations.
