In order to obtain more knowledge on the mechanism of action of mycophenolate mofetil (MMF), a drug used in the prevention of acute rejection after kidney and heart transplantation, we studied the effects of this drug in healthy volunteers. Ten healthy volunteers were treated with a single dose of 1 gram of mycophenolate mofetil (MMF). Just before treatment and 1, 2, 6, 12 and 24 hours after administration of the drug blood was drawn to study its effect on the proliferation and the activation of the white blood cells that cause rejection (lymphocytes) and on the production of certain activation hormones (cytokines). The magnitude of inhibition of these lymphocyte functions were compared with the concentrations of MMF measured in blood of the human volunteers at the same time points. We observed that there was a strong correlation between the amount of MMF present on one hand and the inhibition of proliferation and activation on the other hand. The effect on cytokine production was only modest, and not correlated with drug concentrations.