Adenosine deaminase activity (ADA) was assayed in the serum of 137 HIV positive subjects: 131 intravenous drug abusers, 3 female partners of HIV positive abusers, 1 son of HIV positive abuser and 2 blood-transfused patients, subdivided into the following groups: 73 asymptomatic, 15 with ARC, 37 with LAS, 5 with L-AIDS and 7 with AIDS. Results show an increase of ADA activity in 100% of L-AIDS group, in 64% of AIDS group, in 62% and 42% of LAS and ARC groups, respectively, and in 36% of the asymptomatic groups. These findings must be confirmed with wider series in order to be further and better evaluated. According to relative substrate specificities, optimal pH and Km, enzymatic activity can be attributed principally to the ADA2 isoenzyme. The probability that ADA2 originates exclusively from the Monocyte-Macrophage cell system (MoMaCS) which actively releases this enzyme in the presence of live parasites in the cells' interior, is discussed. Moreover, it was hypothesized that in the MoMaCS the enzyme constitutes a microbicidal mechanism independent of the respiratory burst.