A quick, simple and reproducible method for the indirect determination of the population of the absorbed and suspended bacterias in the cultivation media is the Bradford's method. It has been used to evaluate the fraction of bacteria adhered to the mineral during the bioleaching process and the suspended fraction. It was observed that a significant fraction of the bacteria is adhered to the mineral surface. This proportion varies with the environmental conditions, mainly pH, the presence of ferric ions and the way to supply the mineral sulphide. It has also been observed that the fraction adhered to the solid shows a progressive loss of its oxidative activity. The results can help demonstrate that the sulphides leaching is due to the combination of the direct bacterial action on the sulphide mineral surface with the indirect leaching by the ferric ion produced by the bacteria into the solution.