Two distinct isoenzymes of ADA (adenosine deaminase), ADA1 and ADA2, have been found in humans. Inherited mutations in ADA1 result in SCID (severe combined immunodeficiency). This observation has led to extensive studies of the structure and function of this enzyme that have revealed an important role for it in lymphocyte activation. In contrast, the physiological role of ADA2 is unknown. ADA2 is found in negligible quantities in serum and may be produced by monocytes/macrophages. ADA2 activity in the serum is increased in various diseases in which monocyte/macrophage cells are activated. In the present study, we report that ADA2 is a heparin-binding protein. This allowed us to obtain a highly purified enzyme and to study its biochemistry. ADA2 was identified as a member of a new class of ADGFs (ADA-related growth factors), which is present in almost all organisms from flies to humans. Our results suggest that ADA2 may be active in sites of inflammation during hypoxia and in areas of tumour growth where the adenosine concentration is significantly elevated and the extracellular pH is acidic. Our finding that ADA2 co-purified and concentrated together with IgG in commercially available preparations offers an intriguing explanation for the observation that treatment with such preparations leads to non-specific immune-system stimulation.