Dairy cattle and pig manure mixed with sphagnum peat and a little barley straw was composted in a continuously working horizontal drum composting system in three successive series. Changes in microbiological and chemical parameters were monitored during the six days' processing in the drum and the subsequent three months' curing in heaps.The temperature of the composting mass in the middle of the drum varied between 42-45°C; the outfeeds had an initial temperature of 40°C; and within two to three days the heaps had reached a temperature of 65-70°C which was maintained for a few days. Hygienization, as determined by the absence of fecal streptococci, was mostly reached in six days' active composting in the drum. The most abundant bacteria in the feeding materials were mesophilic bacteria. The amounts of these fell slightly during the processing in the drum, but they rose again during the curing period as a whole and remained high, varying from 4.2×108 to 1.7×1010 CFU/gdw. Thermophilic bacterial populations remained fairly constant during the composting, varying between 107 and 7×108 CFU/gdw.Microbial activity in the feeding material, measured in terms of dehydrogenase activity, was typically high during composting, varying from 24.4 to 27.8 μmol TPF/gdw/3 h. During processing in the drum it remained high, but during curing it showed a decreasing trend, and by the end of the experiment it had stabilized at a low level in all the experimental heaps, varying from 0.71 to 1.53 μmol TPF/gdw/3 h, indicating maturation of the compost.The ratio C/Nfinal to C/Ninitial fell to 0.66-0.77 and then to 0.55-0.74 after two months' and three months' curing, respectively. The average loss of C, calculated from C/P data, was during the drum composting 12%, and 39% when calculated for the mature composts. The average N loss was 16% in the drum, but only 5% when calculated for the mature compost. The total amount of water-soluble mineral N fell from c. 450-500 mg/100 gdw in the infeeds to c. 90-160 mg/100 gdw in the composts after the 1st turn. Thereafter it gradually rose again to the initial level. During composting the proportion of NH4-N declined from 100% to below 20%. PO4-P concentration showed an increasing trend during the whole composting process, and stabilized after two to three months' curing to a level of 400 mg P/100 gdw.