Long term measurements were carried out in aprrallel on two bio-filters under practical conditions to test their reduction potential for ammonia and odor emissions. Ther average efficiency of ammonia reduction was calculated for the first measuring period was about 15% at biofilter 1 and 36% at biofilter 2. The average efficiency of odor reductionwas calculated for the first measuring period was about 78% and 80% at biofilter 1 and 2 respectively. The efficiency of ammonia reduction was mainly influenced by the air flow rate. The odor reduction was mainly inluenced by the odor concentration before the filterbed. The cleaning efficiency of the biofilters could be improved by increasing the moisture content of the filterbed material from 20 to 40% or from 20 to 50%