The in vitro synthesis by mouse hepatocytes of the major acute-phase reactant, serum amyloid P-component (SAP), was induced either by inflammatory macrophages or by the addition of monokine(s), including IL 1. A single cell assay for enumerating SAP-secreting hepatocytes was developed. An increase in the frequency of SAP-synthesizing hepatocytes was found during the acute phase of inflammation. Macrophages elicited with a sterile inflammatory agent, when cultured with hepatocytes, both induced new SAP synthesis by the hepatocytes and increased the number of SAP-producing hepatocytes by sevenfold. Inflammatory macrophage culture supernatants induced new SAP synthesis in hepatocytes; however, the inducing activity did not correlate with the IL 1-dependent thymocyte-proliferating activity. Purified IL 1 alone increased SAP production without increasing the number of hepatocytes secreting SAP. A mixture of purified IL 1 with non-IL 1 monokines both increased the number of SAP synthesizing hepatocytes and the amount of SAP secreted per cell. Two non-IL 1 monokines of 70 to 80 Kd and 30 to 40 Kd were responsible for hepatocyte induction. The inducing activity was not neutralized by anti-mouse IL 1 antibody. IL 1 did contribute to the acute phase response by inducing more SAP synthesis per hepatocyte. The findings suggest that both the induction of nonsynthesizing hepatocytes into new SAP synthesis and the enhancement of the amount of SAP produced per hepatocyte are responsible for the increase in blood levels of SAP during the acute phase of inflammation.