  We are carrying out a program of high signal to noise optical spectroscopy of
BL Lacs with unknown or tentative redshift. Here we report some preliminary
results. New redshifts are measured for PKS0754+100 (z=0.266) and 1ES0715-259
(z=0.464) . From lineless spectra of PG1553+113 and PKS1722+119 we set a lower
limit of z>0.3 for both sources. In two cases (UM493 and 1620+103) stellar
spectra indicate a wrong classification.
