Mononuclear phagocytes are essential cellular mediators of both acute and chronic inflammatory responses. In addition to producing substances that mediate tissue injury directly, such as proteolytic enzymes and oxygen radical species, mononuclear phagocytes can secrete proteins involved in the activation and recruitment of inflammatory cells. One of the major inducible polypeptides secreted by mononuclear phagocytes is macrophage inflammatory protein 1 (MIP-1). Native MIP-1 is a protein with leukocyte chemotactic and stimulatory activity. MIP-1 consists of two highly homologous peptides, MIP-1 alpha and MIP-1 beta. We now characterize the expression of MIP-1 alpha from human peripheral blood monocytes (PBM) and identify the T-lymphocyte product interleukin-4 (IL-4) as an important regulator of MIP-1 alpha expression from PBM. In initial experiments, we demonstrated the production of MIP-1 alpha from lipopolysaccharide (LPS)-, interleukin-1 (IL-1)-, and phytohemagglutinin (PHA)-stimulated PBM. IL-4 inhibited the production of MIP-1 alpha from LPS-, IL-1-, and PHA-challenged PBM by 63, 81, and 88%, respectively. The suppressive effects of IL-4 were operative at the level of MIP-1 alpha mRNA, which was reduced in a dose-dependent fashion by IL-4. The suppression of MIP-1 alpha mRNA by IL-4 was observed within a narrow temporal window and was dependent upon the de novo synthesis of a protein intermediate. As determined by mRNA stability studies, IL-4 decreased steady-state levels of MIP-1 alpha mRNA, in part, by accelerating MIP-1 alpha mRNA decay.(ABSTRACT TRUNCATED AT 250 WORDS)