Endotoxin (lipopolysaccharide [LPS]) is a potent activator of a number of inflammatory genes in blood leukocytes, including interleukin-1 (IL-1). Blood leukocytes isolated from patients with septic shock fail to produce IL-1 in response to LPS, a phenomenon known as endotoxin tolerance. To study the regulation of IL-1 expression in endotoxin-tolerant cells, the protein phosphatase inhibitor okadaic acid was used to examine the effects of protein phosphorylation on IL-1 gene expression. We found that endotoxin-tolerant cells produced normal levels of IL-1 when protein phosphatases were inhibited. In the human pro-monocytic cell line THP-1, okadaic acid increased mRNA accumulation and synthesis of IL-1 protein. Normal and endotoxin-tolerant THP-1 cells accumulated IL-1 mRNA and protein with similar delayed kinetics. Okadaic acid stabilization of IL-1 mRNA appears to be the primary mechanism through which endotoxin-tolerant cells accumulate IL-1 mRNA and protein. Endotoxin-tolerant cells were unable to activate transcription in response to okadaic acid. However, the transcription factor NF-B, which is known to be involved in IL-1 expression, was translocated to the nucleus in both normal and endotoxin-tolerant cells after treatment with okadaic acid. These studies revealed that protein phosphorylation can affect gene expression on at least two distinct levels, transcription factor activation and mRNA stability. Endotoxin-tolerant cells have decreased transcription activation potential, while IL-1 mRNA stability remains responsive to protein phosphorylation.