The stresses for macroscopic plastic flow and critical stages of fracture, fracture toughness and hardness of sintered plus forged T1 high speed steel were determined. The results are compared to similar data for sintered, sintered to closed porosity plus hot isostatically pressed a nd electroflux refined (EFR) alloys of comparable composition. EFR meltstock, with addition of 0.6 wt% Mo, was water-atomized in a 200 kg unit which incorporated ceramic filters and an argon shroud to ensure maximum cleanliness. The powder was sieved, <125 mu m, vacuum annealed, blended, isostatically compacted and vacuum sintered and hot forged to produce a 300 kg billet. Mechanical properties were determined in four-point bending of heat-treated beam specimens. Most samples showed evidence of macroscopic plastic flow, up to similar to 1%, beyond a stress of similar to 1.8 GPa, sigma(Y). Using surface replica microscopy, crack nucleation was detected at stresses sigma(N), between 0.5 and 0.9 sigma(Y), and subcritical short crack growth, at stresses generally larger than sigma(Y). Fracture, from crack nuclei associated (only) with fractured M6C carbides, took place at stresses, sigma(F), in the range 1.4 to 3.0 GPa Macroscopic fracture toughness, K-IC, was in the range 17-24 MPa m(1/2) and, like sigma(N) and sigma(F), appeared to depend sensitively on the tempering temperature. The most attractive combination of properties, for the overtempered, 580 degrees C, structure at HV50 similar to 750 appears to be : sigma(Y) approximate to 1.9 GPa, sigma(F) approximate to 2.8 GPa, K-IC approximate to 23 MPa m(1/2). These values are comparable to those for EFR aerospace quality T1 high speed steel.