To promote a bioactive surface on cobalt based alloys a biomimetic method was used. This method consists of an alkali treatment with a sodium hydroxide solution followed by a heat-treatment of the biocompatible alloys. The treated substrates were immersed in a simulated body fluid with ion concentrations nearly equal to those of human blood plasma. Two different cobalt based alloys, cast and wrought Vitallium(R), were used. The cast Vitallium(R) was obtained by investment casting technique. The effects of quality surface and temperature of the heat treatment (400 and 600 degreesC) were also evaluated. The reaction interface was investigated by using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDX). An apatite layer was formed on the surface of all the materials treated at 600 degreesC after 16 days of immersion in SBF However, a more homogeneous and thicker layer was obtained for the as-cast alloy in comparison with the wrought Vitallium(R). The polishing of the samples has also a considerable effect on the thickness of the apatite layer, being this higher for the finer polished samples. No changes on the surface of the materials treated at 400 degreesC were observed.