Adenosine deaminase (ADA) has been assayed in plasma, erythrocytes, and lymphocytes from 29 patients with haematological and autoimmune diseases. ADA activity was uniformly low in erythrocytes and lymphocytes from patients with non-Hodgkin lymphoma and multiple myeloma (p < 0.001). High levels of ADA activity was found in plasma, erythrocytes, and lymphocytes from patients with myeloid leukemia (p < 0.001). ADA was high in plasma but low in erythrocytes and lymphocytes from patients with autoimmune diseases treated with immunosuppressive drugs (p < 0.05). 4 adults with congenital immunodeficiency showed decreased ADA activity. In the control group of normal blood donors we found a 34-year-old female with low ADA activity in plasma, erythrocytes, and lymphocytes without any immunological abnormalities. This is the 3rd case of a healthy individual deficient for ADA. 1 patient with Osier’s disease and high ADA activity in erythrocytes showed the importance of the purine salvage enzyme not only in lymphocytes.