A novel system for exposing purified human mononuclear leukocyte subsets or any cultured cell to inhalational anesthetic agents has been devised. Monocytes and lymphocytes are purified by counter-current centrifugal elutriation and put into culture vessels with and without appropriate functional activators. The culture vessels are placed into one of four anesthetic agent exposure chambers, each containing a different concentration of the anesthetic agent to be tested. The system that delivers the anesthetic agent to the culture vessels is essentially the same as the one used in the operating room; the actual levels of anesthetic agent delivered are monitored. In this report, we present evidence that halothane can be successfully tested using the above-cited experimental design; this agent substantially inhibits the secretion of interferon by human mononuclear cells. The ability of monocytes to secrete alpha interferon in response to polyinosinic: polycytidylic acid (poly I:C) was significantly depressed following 4 h in vitro exposure to increasing concentrations of halothane; the secretion of gamma interferon by lymphocytes in response to phytohemagglutinin (PHA) was also depressed, although to a lesser degree. The system presented should allow for the in vitro exploration of the effects of inhalational agents on purified human leukocyte subset functions as well as for the analysis of the effect of these agents on monocyte/lymphocyte interactions.