The effects of usual or recommended rates of application of five organic amendments (24 t/ha.yr of MSW compost, sewage sludge, and ovine manure, 2.4 t/ha.yr of vermicompost, and 100 l/ha.yr of a commercial humic acids solution) on the microbial biomass content and level of selected enzymatic activities (dehydrogenase, alkaline phosphomonoesterase, phosphodiesterase, arylsulphatase, and urease) in soil, were evaluated in a long-term field experiment. Four and five years after the beginning of applications, neither vermicompost nor the humic acids solution showed any significant effect, suggesting that rates recommended by the producers and imposed by their high prices are too low to be useful. Owing probably to more realistic application rates, the highest values of all parameters studied were found when organic residues were applied, effects reaching statistical significance in most cases, MSW compost was the product yielding greatest enhancement of soil enzymatic activity, ovine manure and sewage sludge giving lower, but comparable results. Most of the parameters studied were found to be highly correlated, indicating a balanced enhancement of soil biological activity after the application of organic residues.