A method to promote a bioactive surface on the cobalt base alloy ASTM F75 was tested. A set of cylindrical samples was obtained using the investment casting technique and one of the flat surfaces of each sample was polished. The samples were packed in wollastonite powder and then heat treated for 1 h at 1220°C. To characterize the in vitro bioactivity, a set of heat treated metallic specimens was immersed in a simulated body fluid with an ionic concentration nearly equal to that of human blood plasma (SBF) for 7, 10 and 21 days. To evaluate the effect of increasing the concentration of the simulated body fluid on the bioactivity of the material, a second set of heat treated samples was immersed in a more concentrated solution (1.5 SBF). In both cases the solution was renewed every 7 days. After heat treatment, fine agglomerates homogeneously distributed, containing O, Ca, Si and Al were observed on the metallic surface. After immersion of the samples in simulated body fluids, a ceramic layer containing Ca and P was formed on all the samples. A thicker layer, identified as apatite by XRD, was formed on the samples immersed for 21 days in SBF and as early as 7 days in 1.5 SBF. This may indicate that the heat treated material is useful for bone replacement and tissue regeneration under highly loaded conditions.