Human monocytes may synthesize simultaneously both the agonist IL-1 beta and a specific receptor antagonist of IL-1 (IL-1ra). These studies examined whether monocyte production of these two structurally related cytokines was regulated differently. IL-1ra and IL-1 beta protein levels in cell supernatants and lysates were measured with specific ELISA. Relative steady-state mRNA levels, relative transcriptional rates as determined by the nuclear run-on technique, and mRNA stability were all assessed using specific cDNA probes. Monocytes were stimulated with LPS alone, adherent IgG, or both LPS and adherent IgG. Monocytes stimulated with LPS produced near equivalent amounts of IL-1ra and IL-1 beta proteins over 22 h; relative steady-state mRNA levels paralleled the protein levels. In addition, LPS-induced monocytes exhibited enhanced rates of transcription for both IL-1ra and IL-1 beta, in comparison to adherent control cells without LPS. mRNA half-lives in LPS-induced monocytes also were similar for IL-1ra and IL-1 beta. Monocytes cultured on adherent IgG exhibited a low level of IL-1 beta transcription with an absence of protein production. In contrast, adherent IgG led to a high and prolonged rate of IL-1ra protein production. Furthermore, monocytes cultured on adherent IgG exhibited a specific induction of IL-1ra transcription and a marked prolongation in IL-1ra mRNA stability. However, LPS in a high concentration, 1 microgram/ml, reversed the IgG induction of IL-1ra production by decreasing both transcriptional rate and mRNA stability. These results indicate that production of IL-1ra by human monocytes is characterized by different patterns of regulation in comparison with IL-1 beta. LPS induces production of both proteins whereas adherent IgG stimulates only IL-1ra production. The effects of IgG and LPS on induction of IL-1ra production in human monocytes are mediated at both transcriptional and post-transcriptional levels.