IL 1 is a major immunoregulatory molecule produced by macrophages, and it appears to be the molecular orchestrator of nonspecific host defense mechanisms against a variety of environmental insults. Many investigators have used artificial agents to stimulate macrophages to produce IL 1. We now report production of large quantities of IL 1 after a physiologic stimulus. The Lyme disease spirochete, recently isolated and adapted for growth in vitro, was used to stimulate P388D1 cells or human peripheral blood monocytes. Spirochetes were added to confluent macrophage cultures in serum-free RPMI at a ratio of 10:1. The release of IL 1 was dose-dependent. The 24-hr supernatant IL 1 activity was determined by using the thymocyte Con A co-mitogenesis assay. Activity was not due to an endotoxin on, or produced by, the spirochete. A polymyxin B affinity column failed to remove activity, and polymyxin B in the spirochete-macrophage culture had no effect on IL 1 production. Supernatants were collected, were concentrated, and were subjected to size exclusion HPLC. Three areas of activity were found in P388D1 cell supernatants (Mr greater than 60,000, 40,000, and 20,000), whereas two peaks (Mr 23,000 and 13,000) were found in human monocyte supernatants. The Mr 20,000 and 13,000 peaks from murine and human cell supernatants, respectively, were subjected to SDS-PAGE and were shown to be single bands (Mr 12,400 for the mouse IL 1 and Mr 13,500 for the human IL 1). Isoelectric focusing of column-purified IL 1 preparations showed two different pI in both human (pI 7.25 and 4.4 to 5) and murine (pI 7.25 and 5.55) IL 1. Fibroblasts cultured with murine or human IL 1 preparations demonstrated both an increase in secreted collagenase and increased cell proliferation. Thus, a physiologic stimulus and simple biochemical techniques produce large amounts of very pure mouse or human IL 1. That this IL 1 is produced by Lyme disease spirochete-stimulated macrophages may explain some of the clinical manifestations of Lyme disease.