We evaluated serum total adenosine deaminase, its isoenzymes adenosine deaminase-1 and adenosine deaminase-2, and cytidine deaminase activities in 24 patients with active systemic lupus erythematosus, and in 26 healthy control subjects, and found the means+/-SD values to be 21.38+/-5.96 IU/l, 3.74+/-2.12 IU/l, 17.72+/-5.02 IU/l and 17.89+/-4.62 IU/l, respectively in the patients, and 14.97+/-4.71 IU/l, 4.01+/-1.35 IU/l, 10.91+/-3.91 IU/l and 7.39+/-3.97 IU/l, respectively in the control subjects. When compared to the healthy controls, serum total adenosine deaminase, adenosine deaminase-2 and cytidine deaminase levels were significantly higher (p<0.001) in systemic lupus erythematosus patients, but the decrease of adenosine deaminase-1 level was not statistically significant (p>0.05). The increased adenosine deaminase-2 may be of macrophage origin. It closely correlated with clinical signs of active systemic lupus erythematosus. The membranes of polymorphonuclear neutrophils may be damaged, and cytidine deaminase may be released into serum.