The relationship between quantity and type of nonmetallic inclusions and critical mechanical properties was determined for the high-strength steels 4340 and 300M. Quantitative television microscope scanning data and various petrographic and microprobe methods were used to study the inclusions. Inclusion statistics showed that improved cleanliness was obtained in ascending order of (1) electric furnace air melted, (2) Dortmund–Horder degassed, (3) vacuum-arc–remelted and (4) electron beam hearth refined steels. Studies in the electron beam melted material were made on 1000 lb heats, whereas the remaining data were obtained on large-scale production heats. The properties for both 4340 and 300M improved in a similar manner with cleanliness. Transverse reduction of area was the property most influencd by inclusion content. Fatigue and impact properties were also affected, though to a lesser degree. Plane strain fracture toughness was relatively little affected by inclusion content and, in fact, longitudinal and transverse properties were substantially the same in direct contrast to reduction of area, impact, and fatigue properties. KIc is thus not a meaningful parameter for quality control measurement in these high-strength structural steels. While we realize that other factors, such as microsegregation, affect properties, we conclude that for steels processed according to specified practices, the inclusion content is the most important variable influencing critical mechanical properties.