The effects of a commercial inoculum (MicroGest 10X, Brookside Agra L.C.) on the field-scale composting of the source-selected organic fraction of municipal solid wastes (OFMSW) have been studied by following routine parameters of the composting process (temperature, oxygen content and moisture) and biologically-related tests such as the respirometric index and the maturity grade. The inoculum was added to composting piles of OFMSW at different levels: control (no added inoculum), treatment A (10(5) CFU g(-1) of OFMSW), treatment B (10(6) CFU g(-1) of OFMSW) and treatment C (10(7) CFU g(-1) of OFMSW). The inoculum selected produced a significant acceleration of the composting process with high levels of biological activity in the thermophilic phase. In terms of the acceleration of composting and economy the optimal treatment was B, which produced a reduction of approximately half of the total composting time. Treatment C did not improve significantly the results obtained with treatment B, whereas treatment A has little effect on the composting of OFMSW when compared with the control experiment. Respirometric index (determined at 55 degrees C) and maturity grade appeared to be the most reliable tests to follow the biological activity of the composting of OFMSW. On the other hand, routine parameters such as temperature, oxygen content and moisture showed no significant differences among the different inoculation levels tested in the composting process. (c) 2005 Society of Chemical Industry.