Parke-Davis developed the first bacterial vaccine, and the company was thus known as a pioneer in the field of vaccinology. It was also among the five firms contracted to manufacture the original Salk killed-virus vaccine. A combination of the DPT and polio vaccines, called Quadrigen, was developed in 1954 and approved in 1959. Quadrigen was later removed from the market in 1968 after a series of lawsuits pertaining to adverse effects in vaccinated children. Parke-Davis also produced the broad-spectrum antibiotic chloramphenicol, which was a blockbuster product before the discovery of its association with aplastic anemia.