The effect of a variety of residual elements on the tensile and toughness properties of special carbon and low alloy engineering steels has been determined. The major objective of this work has been to identify the extent to which residual element contents can be allowed to rise without infringing the specification and in-service performance requirements of these steels. In C-Mn and C-Mn-B steels, the main effect of the residual elements, chromium, molybdenum, nickel and copper, is to raise hardenability and tensile strength with a concurrent reduction in ductility. The effects on toughness are dependent upon the microstructural changes accompanying the increase in hardenability and can be either beneficial or detrimental. A statistical approach has been adopted in quantifying the influence of these elements on hardenability. In many cases, naturally occurring levels of chromium, molybdenum, nickel and copper can be used to advantage as the basis for providing cheaper alternatives to low alloy steels. However, it might be necessary to compensate for very high residual element levels by reducing the carbon and/or managese levels in order to maintain the currently specified hardenability limits. In low alloy steels, phosphorus, arsenic and tin were found to exhibit the greatest influence on toughness and this effect was most detrimental in the Cr and Ni-Cr steels. The presence of molybdenum reduced the susceptibility to embrittlement. In most of the low alloy steel grades examined, it has been concluded that there would be no foreseeable violations of property specifications, even if the residual content were allowed to rise much above the current levels. Only in certain cases, e.g. 815H17, was it concluded that the phosphorus, arsenic and tin contents must be maintained at the current levels produced by electric arc steelmaking in order to satisfy user requirements.