B iosynthesis and secretion of alpha-1-proteinase inhibitor (alpha 1 PI) has been demonstrated in primary cultures of human mononuclear phagocytes, making it possible to study regulation of alpha 1 PI in normal (PiMM) and homozygous-deficient (PiZZ) individuals. In this study, expression of alpha 1 PI by blood monocytes, bronchoalveolar, and breast milk macrophages decreased during 1 wk in culture whereas expression of other secreted proteins increased. The addition of crude supernatants from mitogen-stimulated peripheral blood mononuclear cells to confluent monolayers of mononuclear phagocytes after 1 wk in culture resulted in a 2- to 2.5-fold increase in alpha 1 PI expression. The increase in alpha 1 PI expression was dose- and time-dependent, and involved a mechanism acting at a pretranslational level as shown by an increase in specific messenger RNA content corresponding to the increase in synthesis and secretion of alpha 1 PI. Although alpha 1 PI was expressed in native form and in forms complexed with serine protease by monocytes early in culture, it was expressed in its native form alone when monocytes were incubated with the lymphokine after 1 wk in culture. The regulating factor had the characteristics of a polypeptide and was derived from T lymphocytes, but it was not interferon-alpha, -beta, -gamma, or interleukin 2. This lymphokine also stimulated synthesis of alpha 1 PI in monocytes of homozygous-deficient PiZZ individuals, but had minimal effect on secretion, thereby increasing the intracellular accumulation of the inhibitor and exaggerating the defect in secretion of alpha 1 PI in these individuals. Regulation of mononuclear phagocyte alpha 1 PI expression by a lymphokine provides a model for further analysis of the effect of enhanced synthesis on a defect in posttranslational processing/secretion and for analysis of differential regulation of protease and inhibitor expressed in the same cells.