IL-10 is a pleiotropic cytokine produced by monocytes and T cells that has potent inhibitory effects on monocyte/macrophage function. Because monocytes and macrophages are capable of releasing the C-C chemokine, macrophage inflammatory protein-1 alpha (MIP-1 alpha), which is a potent chemoattractant for activated T cells, we studied the effects of IL-10 on the expression of MIP-1 alpha in these cells. Low levels of MIP-1 alpha were detectable in resting monocytes and macrophages. Both LPS (1 micrograms/ml) and IL-1 beta (10 ng/ml) induced the expression of MIP-1 alpha mRNA and the release of MIP-1 alpha protein from these cells. The addition of exogenous human rIL-10 inhibited induced MIP-1 alpha mRNA expression as well as the release of MIP-1 alpha protein measured after 24 h. This inhibition was significantly higher in monocytes compared with alveolar macrophages. In monocytes, IL-10-induced inhibition of MIP-1 alpha was only partially accounted for by alterations in mRNA stability and was dependent on de novo protein synthesis. In the presence of an anti-human IL-10-neutralizing Ab, the release of MIP-1 alpha induced by LPS and IL-1 beta was further enhanced in monocytes but unchanged in alveolar macrophages. MIP-1 alpha mRNA was also increased in monocytes. There was no detectable release of IL-10 from alveolar macrophages after LPS or IL-1 beta in contrast to modest amounts released from monocytes. Thus, IL-10 is an inhibitor of the induced transcription of MIP-1 alpha mRNA and of the release of MIP-1 alpha protein, with a greater effect on monocytes as compared with alveolar macrophages. IL-10 may indirectly regulate effects on cells such as activated T lymphocytes partly through the inhibition of MIP-1 alpha expression from monocytes and macrophages.