In the present study we demonstrate that enzymatically active l-Dopa decarboxylase (DDC) is expressed in U937 cells, an important model of human macrophage function. The mRNA coding for the enzyme was found to be of the neural-type. DDC was detected in both the soluble and membrane fractions. The active molecule is released from the membrane fraction in the absence of detergent. Furthermore, exposure of U937 cells to carbidopa and dopamine considerably affected cell viability. The expression of enzymatically active DDC shows the endogenous production of dopamine in U937 cells and raises new questions about the enzyme's involvement in immune responses.