3C48 was the first of many faint, starlike quasi-stellar objects which later were named quasars.
3C48 was the first source in the Third Cambridge Catalogue of Radio Sources for which an optical identification was found by Allan Sandage and Thomas Matthews in 1960 through interferometry. Jesse Greenstein and Thomas Matthews found that it had a redshift of 0.367, making it one of the highest redshift sources then known. It was not until 1982 that the surrounding faint galactic "nebulosity" was confirmed to have the same redshift as 3C48, cementing its identification as an object in a distant galaxy. This was also the first solid identification of a quasar with a surrounding galaxy at the same redshift.