Cell-free extracts of nitrate-grown Aspergillus terricola catalyze the hydrolytic deamination of adenosine to inosine at maximum rate at pH 6.5 and 50 degrees C. Incubation of the extracts at 60 degrees C for 30 minutes caused about 66.7% loss in activity. Results indicated the involvement of SH groups in the catalytic site of adenosine deaminase. Frequent freezing and thawing of the enzyme preparation for three days (3 times) resulted in about 47% loss in activity. The enzyme is also inhibited by EDTA indicating that adenosine deaminase is a metaloenzyme. MgCl2 and CoSO4 had a remarkable activating effect, whereas MnCl2 showed a slight inhibitory effect on enzyme activity. The apparent Km value was calculated for adenosine and found to be 6.66 x 10(-3) M, which indicates the greater affinity of adenosine deaminase for adenosine.