In Chile, almost 100% of the urban wastewater will be treated by the end of 2010, which implies the generation of large volumes of biosolids that will need to be disposed to avoid an environmental problem. An interesting alternative to face the accumulation of these residues is their application in agriculture as soil amendment and to complement the soil fertilization. This constitutes an interesting alternative of disposal and recycling of these residues instead of a simple accumulation. On the other hand, continuous biosolid application may have a negative impact on the environment since it may lead to heavy-metal accumulation in soil and, from there, some metals can be uptaken by plants and/ or be leached to freatic levels. In order to determine the availability of metals for crops, sequential extraction procedures have been proposed to provide an estimate of the different associations of heavy metals with the various soil components. To this end, the Community Bureau of Reference (BCR) has proposed a three-stage sequential extraction procedure which has been widely applied in studies of trace-metal fractionation in sediments, soils, and biosolids. The purpose of this study was to determine the effect of biosolid amendment on Cu, Cr, Ni, Pb and Zn extractability from the soil components, after a 60-days incubation period, through the BCR procedure with some modifications controlled through the use of the reference material CRM 483. Soil samples were collected from the surface soil layer (0-20 cm) at four different sites close to the city of Santiago, Chile. These were amended with biosolids at the rates of 15 and 30 Mgha-1 and incubated for 60 days under controlled conditions of moisture and temperature. Domestic biosolids, one taken on summer time and other on winter time, were obtained from the monofill at one of the Santiago wastewater treatment plants. Biosolid and soil sequential extraction were performed according to the procedure recommended by BCR, which considers three extractions to obtain an acid-soluble fraction (exchangeable, carbonates), a reducible fraction (iron /manganese oxides), an oxidizable fraction (organic substance and sulphides). Total trace metal contents found were below the limits allowed by the Chilean regulations for biosolids. The biosolid application increased the total content of all the metals under study. In general, the metals were found predominantly as reducible and oxidizable fractions. Biosolid incorporation at the rate of 30 Mg kg-1 produced an increase in Zn availability in all the soils and Cu in some of them, while Cr, Ni and Pb showed no significant variations. The incubation process did not show remarkable effects on metal distribution compared with initial conditions. A two-level factorial design was satisfactorily applied to assess the effect of biosolid application. Acknowledgement: Founded by FONDECYT, Project Nº 1050288.