Adenosine is known to affect the immune system through its effects on lymphoid cells. In the human lymphocyte adenosine together with homocysteine can be converted to S-adenosylhomocysteine (SAH) by SAH hydrolase (EC 22.214.171.124) . Elevation of adenosine concentrations in cells of various origins leads to accumulation of SAH, which is a potent inhibitor of several transmethylation reactions [2-4]. Both for diagnostic and fundamental purposes we have measured the SAH-hydrolase activity in human T-lymphoid cells with a procedure that requires less material than the conventional methods. Here we present a simple radiochemical microassay for the SAH-hydrolase activity in about 12000 human peripheral T lymphocytes or human thymocytes.