This paper discusses the technological and public health context of the 1921 discovery and subsequent development of the anti-knock gasoline additive tetraethyl lead in light of recently released General Motors correspondence files. The discovery has long been seen as a milestone of systematic research and a vital turning point in the development of modern high compression engines. The documents show that the choice of tetraethyl lead over other alternatives took place within the context of a complex controversy.After leaded gasoline entered the market in 1923 - 24, a fatal refinery accident drew attention to the poisonous nature of the full strength additive and the potential public health risk from fuel containing the diluted additive. Public health advocates claimed that alternatives existed, but industry insisted that tetraethyl lead was the only additive that could be used. The controvery was never resolved because, until the early 1990s, virtually no primary documentary material was available in public archives. Recently released documents support the idea that the original motive for research leading to leaded gasoline was to fortify G. M. against oil shortages (then expected by the 1940s or 1950s) and raise engine compression ratios fo facilitate a transition to alternative fuels (particularly ethyl alcohol). This original motive was not strong enough to shift the balance toward alternative anti-knocks and was apparently discarded when oil supplies proved to be plentiful in the mid-1920s.