QUASARS are the most luminous objects in the Universe. It has been speculated that they are the visible evidence for accretion of gas onto supermassive black holes that reside at the centres of host galaxies. Direct observational confirmation that quasars reside in the centres of galaxies has been hard to obtain, because atmospheric turbulence usually scatters the quasar light sufficiently to swamp the signal from the fainter surrounding galaxy. Despite the difficulties, however, many attempts have been made to observe the host galaxies(1-17), although the results have not been definitive(18). Here we report observations of four quasars, made with the refurbished Hubble Space Telescope. In all four cases the quasars reside in luminous elliptical galaxies with very close companions. This is in contrast to recent work(19,20) in which host galaxies were not observed (in a sample of quasars that has one in common with ours). The elliptical galaxies are featureless, but the presence of close companions is suggestive of continuing interactions.