1. The effect of lithium on phagocytic activity of polymorphonuclear leucocytes (PMNL) has been investigated by measurements of glucose-6-phosphate dehydrogenase (G6PD), NADPH oxidase and myeloperoxidase (MPO) both in lithium treated rats and lithium treated infected rats. 2. The results have been compared with two control groups, one of which was without lithium treatment and the other was only infected. 3. In the first experimental group increased activities of these enzymes have been observed, while in lithium-treated infected rats there was a decrease in the activities of the same three enzymes. 4. It is proposed that defense mechanisms against infection fail during the lithium treatment.