Hubble Space Telescope ( HST ) WFPC-2 I -band ( F814W ) images of three X-ray selected BL Lacertae objects ( MS 1221.8 + 2452 , MS 1407.9 + 5954 , & MS 2143.4 + 0704 ) reveal that each of these BL Lac objects is well-centered in an extended nebulosity that is consistent in brightness and morphology with being light from an elliptical galaxy at the previously reported redshifts of these BL Lac objects .
Each of the detected host galaxies have radial surface brightness profiles that are well fit by a De Vaucouleurs law with effective radii of between 3 to 12 kpc ( H _ { 0 } = 50 km s ^ { -1 } Mpc ^ { -1 } , q _ { 0 } = 0 ) .
The absolute magnitudes of the host galaxies fall in the range -24.7 < M _ { I } < -23.5 , in the range of luminosities determined for other BL Lacertae object host galaxies .

In addition to allowing the measurement of the host galaxy magnitudes and radial surface brightness profiles , the HST images allow a search for substructure in the host galaxies and the presence of close companion galaxies at spatial resolutions not yet achievable from the ground .
While no evidence was found for any “ bars ” or spiral arms , “ boxy ” isophotes are present in the host galaxy of at least one of the three objects observed as part of this study ( MS 2143.4 + 0704 ) .
The apparent magnitudes and image properties of the companions of the BL Lac objects are catalogued as part of this work .
The three BL Lacs appear to occur in diverse environments , from being fairly isolated ( MS 1221.8 + 2452 ) to possibly being a member of a rich group of galaxies ( MS 1407.9 + 5954 ) .