It has been found that a 4145 steel containing 0.3 percent lead suffers a loss in ductility when tension tested between 400 and 900 F. A visual analysis of the fractured surfaces from both leaded and nonleaded tension specimens was performed at various ultimate strength levels from 120 to 240 ksi, and the fractures were classified into five types. An electron microscope fractographic study was made on the five fracture types. Fractographs taken from Type A fractures (room temperature to 400 F) showed that fracture by microvoid coalescence (“dimpled rupture”) typified the entire fracture surface. Type B fractures (400 to 600 F) were characterized by a mixture of intergranular regions in an otherwise transgranular failure. Each group of intergranular grain facets contained an inclusion. For a Type C fracture (600 to 650 F) replicas taken from a cluster of small “fish eyes” showed both intergranular and transgranular modes of failure. Again, inclusions were present on the intergranular grain facets. Type D fracture (650 to 900 F) was almost 100 percent intergranular at the origin of the single flat fish eye. The percent intergranular decreased as the crack grew until it was 100 percent transgranular near the extremity of the fracture at the far side of the fish eye. Once again inclusions were present on the intergranular facets. Due to the fact that the Type E fracture occurs at high temperature (above 900 F), the surfaces were badly oxidized but appear to have been formed by a dimpled rupture process. The association of inclusions with the intergranular fracture regions was deemed significant since the lead was always found in an envelope surrounding the inclusions. The results of this study are compatible with a “liquid metal embrittlement” interpretation of the loss in ductility at elevated temperature.