The effect of the addition of the organic fraction from a fresh municipal solid waste on the regeneration of a degraded soil from the Mediterranean area of Spain was studied. Sufficient waste to increase the organic matter of the soils by 0.5%, 1%, 1.5% and 21% was applied to a field in plots with a 10% incline. Forty months after treatment, increases were observed in organic matter, available nitrogen, water soluble carbohydrates and compounds determined by the Folin method (polyphenols and proteins) in all the treated plots as compared to the control. Changes were the greatest in the plot whose organic matter had been increased to 1.5%). An increase in enzymatic activity was also noticed (urease, BAA hydrolysed by protease, casein hydrolysed by protease, phosphatase, β-glucosidase and dehydrogenase) in the treated soils, particularly those which received higher rates of waste. The extracellular enzymatic activity of the plots treated with higher doses also increased, indicating a regeneration of the soils from a biochemical viewpoint. A positive correlation was observed between the parameters referring to microbial biomass and the enzymatic activities determined in the soils.