Insulin treatment of the rat hepatoma H-35 cells results in a reduced stimulation of acute phase plasma protein gene expression by IL-1- and IL-6-type cytokines. The cell response to insulin appears to involve both stimulatory and inhibitory regulatory mechanisms because a clonal variant line of the H-35 cells has been identified in which insulin increases specifically the IL-1 stimulation of alpha 1-acid glycoprotein (AGP) gene, while still reducing the expression of the other acute phase protein genes. The magnitude of insulin and cytokine effect is dependent upon the proliferation state of the cell culture. One of the genetic targets of the insulin stimulation has been located to the cytokine-response element of the AGP gene and involves a cooperativity with the 5' adjacent IL-1-responsive element. The molecular mechanism of insulin inhibition, however, remains to be identified.